Praise be to Allaah.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) was asked this question and he replied as follows:
If a seeker of knowledge wants to be a scholar of fiqh then he must combine fiqh and usool al-fiqh in order to be erudite and a specialist in that field. Otherwise you may know fiqh without knowing its usool (bases), but you cannot know usool al-fiqh and become a faqeeh without knowing fiqh. In other words, it is not possible for the faqeeh to do without usool al-fiqh or for the usooli (the one who knows usool al-fiqh) to do without fiqh if he wants to learn fiqh. Hence the scholars of usool differed as to whether it is better for the seeker of knowledge to start with usool al-fiqh so that he can build his knowledge of fiqh on that basis, or to start with fiqh because that is needed, i.e., people need to know that with regard to their worship and dealings with others even before they learn usool al-fiqh. The second is more correct and is that which is usually followed.