1) If you are reading then you are not Tweeting or lurking on Facebook.
2) Writers easily become self-referring; that is, we can begin to think of our writing as the standard-bearer of good writing. Reading others' works reminds us that our work not only is not the standard but also that this writer probably has sold more books than we have or ever will.
3) It slows you down because reading good writing often takes time to read. Many times you must penetrate several chapters before the momentum of the book pulls you into it. This develops patience, persistence, and (if you're open to it) an ability to authentically relish the power of simple words rightly placed and the pace of a good story. It will serve well the tenacity it takes to stay with a writing project.
4) You begin to acquire a sense about what makes a book good and what pulls it down. These instincts then arise when you are writing your own narrative.
5) You begin to hear the writers "narrative voice," which is the backbone of every good book. Hearing another writer's narrative voice enables you to scrutinize your own. Without a clear narrative voice, your book-in-process will sink to the bottom of a lonely sea.
Special Tip: Regularly divide your reading between contemporary writers and classics. Time is the true test of a good book. If a book passes the test of time then it was written by someone whose writing you should emulate, or least, appreciate.