Information, perspective, reflection as someone else has said.
Books about golf in Ireland and Scotland, and other places I have not played, open my mind up to other possibilities of golf -both design and destination.
Along the same lines - access. I might never get to play Pine Valley or NGLA, but reading books gives me a better appreciation of those courses.
From books like Anatomy of a Golf Course, I get hit over the head with essential design elements or philosphies, which provides a frame of reference for my impressions of a particular course. Without these primers, I might not fully appreciate or be able to articulate why I like one hole or course but not another. Especially if I don't play the course over and over and allow the impact to sink in. With a frame of reference, I might more fully appreciate why I like it.
Golf books without pictures - either photos or drawings (at least GCA oriented books), are no where as good, imho.
My thoughts anyway.