Much of the time philosophers study what other people take for granted. Philosophy thus involves explanations, arguments, and their critical evaluation. As an attempt at a systematic theory logic has to start somewhere. At the very least, we may find it difficult to construct a systematic account that accommodates all, or even most, of the raw data. Patterns, or forms, of argument are the main object of study.
Formal logic is an indispensable item in the contemporary philosopher's toolkit.
Learning about any systematic discipline improves one's own abilities in thinking systematically. And certainly, the methods and techniques of formal logic are used by some philosophers, mostly those working in philosophy of language and philosophy of science. Two text-books that integrate formal logic into a range of topics in philosophy are Brenner's Logic and Philosophy and Bradley and Swartz's Possible Worlds . University of Notre Dame Press, 1993.