What is the importance of drinking water in sitting position?.
It is permissible to drink both sitting and standing, although drinking whilst sitting is better, and it is what the Prophet (blessings and peace of Allah be upon him) did most often. This has been discussed in the answer to question number 21147.
Ibn al-Qayyim (may Allah have mercy on him) said, explaining the negative effect of drinking whilst standing and the importance of drinking whilst sitting:
Drinking whilst standing has numerous negative effects, one of which is that it does not fully quench the thirst and does not settle in the stomach so that the liver can distribute the nutrients to other parts of the body. Rather it comes to the stomach with speed and force, and there is the fear that it will cause a chill or disturbance, and may pass quickly to the lower parts of the body without moving gradually. All of that is harmful to the drinker, but if he does it occasionally or in the case of need, it will not harm him... End quote from Zaad al-Ma‘aad, 4/229.
Al-Tahhaawi (may Allah have mercy on him) said:
It was narrated that al-Sha‘bi said: Drinking whilst standing is only disliked because it is harmful. And al-Sha‘bi gave the reason for which this is not allowed: because of the fear of either harm and disease that results from that, and nothing else. In forbidding that, the Messenger of Allah (blessings and peace of Allah be upon him) was motivated by compassion for his ummah and sought to tell them to do that which is in their best interests in both religious and worldly terms, as he said to them, “As for me, I do not eat reclining.” That does not mean that it is haraam for them to do that, rather the meaning is that there is some fear for them if they eat reclining. Al-Sha‘bi mentioned the reason why the Messenger of Allah (blessings and peace of Allah be upon him) disliked eating reclining, and it is only because of what happens as a result, such as enlargement of the belly. The same is true of what was narrated from him about the prohibition on drinking whilst standing; it is only because of what may result from that and he disliked it for that reason and no other. End quote from Sharh Ma‘aani al-Aathaar, 4/274
What the believer should do is follow the guidance and Sunnah of the Prophet (blessings and peace of Allah be upon him), whether he knows the wisdom behind it or not; there is nothing wrong with asking about the reason or looking for it so as to increase his faith or to debate, in the way that is better, with those who reject this Sunnah.
And Allah knows best.