I love going to see authors read and talk. First, their approach to writing is often so different than mine. I remember Susan Fromburg-Schaffer talking about how she researched a book for four or five years (once even learning Japanese), but makes no notes at all. Then she writes 20 hours a day and has the whole thing done in a few weeks. Phil Margolin creates a detailed outline that runs nearly 100 pages, then simply fleshes it out for the book. Carol Shields once explained the patterns she followed when writing, but I don't have my notes any longer.
Second, it's just fascinating to see what they are like in person. Clyde Edgerton did like a reader's theatre when he read from one of his book, putting on a baseball cap for one character, and eating an apple when he was a second. But a friend leaned over to me at one point and whispered, "I'll bet he's a drinker." I have no idea if that's true, but I could see it. Charming to the point of recklessness.
Third, you make an author's day just by being there. Buying a book is optional.
Fourth, you never know who you'll run into or what doors will open. Which is why I got to spend a little time with Michael Chabon in 1992 - because he was a friend of Louis B. Jones, and there were only three people in the audience.