Praise be to Allah.
Mankind is in need of Messengers and Prophets from Allah, to light the way and guide them to ways of peace, and to warn them against the ways of evil and corruption. Some of the things for which mankind clearly needs Messengers and Prophets are:
1 - Guidance to Allah, may He be exalted and glorified
Mankind has gone through periods where they did not have Messengers and Messages from Allah for a long time, and so they fell into the traps of illusions and myths, so they prayed to natural phenomena and heavenly bodies, or idols that could neither benefit nor harm them, worshipping them in fear and hope. At the same time, they were subjected to humiliation by those who claimed divinity, such as the Pharaohs and others. Even though there was never a time when at least a few people wondered about Who had sovereignty over the heavens and the earth and instinctively realized that there was a Creator, they still failed to worship Him properly and heed His commands and prohibitions and understand what He wanted them to do and not do, because they had no contact with anybody who could tell that about all that. The wisdom of Allah dictated that He should save the confused and misguided by blessing them with the knowledge of His Divine qualities and Beautiful Names, so He honoured them and all of humanity by sending Messengers who would guide them towards the Creator, as He tells us in the story of Nooh (peace be upon him): "Indeed, We sent Nooh to his people and he said, 'O my people! Worship Allah! You have no other ilaah (god) but Him. Certainly, I fear for you the torment of a Great Day!'" [Al-A'raaf 7:59]. All the Prophets and Messengers, peace and blessings of Allah be upon them, came one after the other, bringing the same Message.
2 - Guidance regarding the Hereafter
One of the reasons why Prophets and Messengers were sent was to tell people about the Hereafter, because if man does not believe in Allah and the Last Day, he will be a slave to his desires, running after material things and following the ways of misguidance. If he is reminded or advised, he will deny that he is answerable for his deeds. Allah has described such people in His words (interpretation of the meaning): "And they say, 'There is nothing but our life of this world, we die and we live and nothing destroys us except al-dahr (the time).' And they have no knowledge of it, they only conjecture." [Al-Jaathiyah 45:24].
So it was the task of the Prophets and Messengers to establish proof of the Last Day, and to explain that the One Who created mankind from nothing is able to bring them back to life after their death, as Allah says (interpretation of the meaning): "Say: 'Allah gives you life, then causes you to die, then He will assemble you on the Day of Resurrection about which there is no doubt. But most of mankind know not." [al-Jaathiyah 45:26]. On that Day, the scales of justice will be set up and scores between oppressors and their victims will be settled. Allah says (interpretation of the meaning): "And We shall set up balances of justice on the Day of Resurrection, then none will be dealt with unjustly in anything. And if there be the weight of a mustard seed, We will bring it. And Sufficient are We as Reckoners." [Al-Anbiya' 21:47]. If it were not for this Reckoning, which we know about only through Prophethood, life would be pointless and a means only for wrongdoing - and this is contrary to the wisdom of Allah, may He be glorified and exalted.
3 - Meeting man's need for religion
So long as man remains sound of nature, clear of mind and pure of heart, his need for religion remains a part of him, and he understands that there has to be an Almighty Creator who created and organized the universe in the best way. Thus there emerges from the depths of his being the urge to submit himself fully to the Creator, to seek His help at times of calamity and distress, and to humble himself before Him, as he feels his great need for Him both at times of ease and of hardship. This is referred to in the Qur'aan (interpretation of the meaning): "So set your face towards the religion of pure Islamic monotheism haneefa (worship none but Allah Alone), Allah's fitrah (pattern) with which He has created mankind. No change let there be in khalq-illah [i.e., the Religion of Allah], that is the straight religion, but most of men know not." [Al-Room 30:30]
According to a hadeeth narrated from the Prophet (peace and blessings of Allah be upon him), "Every new-born baby is born according to the fitrah, then his parents make him into a Jew or a Christian or a Magian (Zoroastrian)" (Agreed upon). How could a man know how to become religious and worship his Creator without Revelation from Allah to tell him about it? There has to be a mediator between Allah and His creation so that mankind can know about the religion which the Creator wants them to follow; this is the role of the Prophets.
4 - Guidance regarding proper ways of dealing with one another
It is well known that man is by nature sociable, and that he has wishes and desires that cannot be achieved except in cooperation with his fellow-man. So it is clear that mankind urgently needs guidance to regulate his relationships with others, to outline the right way of behaving, to resolve disputes and conflicts, to guard against wrong-doing and oppression, and to protect people's rights - lest some people oppress others. Allah says (interpretation of the meaning): "Indeed We have sent Our Messengers with clear proofs, and revealed with them the Scriptures and the Balance (justice) that mankind may keep up justice" [Al-Hadeed 57:24].
5 - Guidance towards sound morals
It is also a part of man's nature that he will strive to fulfil his needs even if he does so by means of oppression and domination. So he urgently needs a deterrent, in the form of belief in Allah and the Last Day, to motivate him to acquire good qualities and praiseworthy morals. These can only be known through the divine laws brought and conveyed by the Prophets and Messengers of Allah, peace and blessings of Allah be upon them.
6 - Achieving justice in accordance with the will of Allah
It is well known that Allah, may He be glorified and exalted, is Fair and Just. He rewards the one who does good as a blessing and favour from Him, and He punishes the one who does wrong as an act of justice on His part.
Allah, by His wisdom and mercy, sent His Messengers, from Aadam to Muhammad (may the peace and blessings of Allah be upon them all) to establish proof (so that people would have no excuse). Allah says (interpretation of the meanings):
"And if We had destroyed them with a torment before this (i.e., Muhammad (peace and blessings of Allah be upon him) and the Qur'aan), they would surely have said, 'Our Lord! If only You had sent us a Messenger, we should certainly have followed Your aayaat (proofs, evidences, verses, lessons, signs, revelations, etc.), before we were humiliated and disgraced.'" [Ta-Ha 20:134]
"Messengers as bearers of good news as well as of warning in order that mankind should have no plea against Allah after the Messengers" [Al-Nisa' 4:165]
Allah, Who is the Most Just of judges, has decreed that He will not punish a nation until He has sent a Messenger to them, as He says: "Whoever goes right, then he goes right only for the benefit of his own self. And whoever goes astray, then he goes astray to his own loss. No one laden with burdens can bear another's burden. And We never punish until We have sent a Messenger (to give warning)." [Al-Isra' 17:15]
The above should explain the extent of mankind's need for Messengers and their Messages, and why their happiness in this world and the next is connected to that.
And Allah knows best.