(a) get a feel for what they would like to write
(b) learn what they like in other fiction
(c) learn what they dislike in other fiction
(d) literally see how a book is put together (layout etc)
(e) be inspired by what they read and feed their own imagination
(f) Agents/publishers will ask you who you read. They like proof you support the industry!
Point (e) is vital. Every writer builds on what has gone before but to be able to do that well, you've got to read well. I like to read outside my own genre and non-fiction so I give my brain a nice eclectic mix to think about as I come up with my stories. Besides which it is huge fun. And isn't that the point of both writing and reading?