The importance of learning Usool al-Fiqh for the one who wants to learn fiqh

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.