Writing is central to the AP English courses and exams. Both courses have two goals: to provide you with opportunities to become skilled, mature, critical readers, and to help you to develop into practiced, logical, clear, and honest writers. In AP English, writing is taught as "process" — that is, thinking, planning, drafting the text, then reviewing, discussing, redrafting, editing, polishing, and finishing it. It's also important that AP students learn to write "on call" or "on demand." Learning to write critical or expository essays on call takes time and practice.
Independent research on the academic benefits of the Advanced Placement English Language and Composition course indicates that not all students receive academic benefits from participating in the course. In a study with a sample size of over 90,000, the authors found that students who took the AP English Language and Composition course did not receive any increase in academic achievement unless they also prepared for and took the AP test. The authors controlled for over 70 intervening variables and found that AP students who took and passed the English Composition and Literature exam had ACT scores that were points higher than non-AP students or AP English students who did not take their course's AP test.  This led the authors to state that AP participation "... is not beneficial to students who merely enroll in the courses ..."  :p. 414