1. Improve your craft. There are usually seminars on everything from idea generation to marketing. Sometimes, presentations cover a more philosophical approach to writing. They're good places to get a taste of different types of writing, like learning to write for children when you're an adult writer. It's not just writers talking about themselves--it's about writers teaching some topic they're knowledgeable about.
Incidentally, it's not always about writing, either. I had about 20 people show up for my presentation on time management for authors. Afterward, other people were asking if they could have my notes.
2. Meeting new editors. Just because you're happy with the editor of your current work doesn't mean you'll publish everything with that house. Michelle Buckman is a good example. Her YA novels have been published by a major Christian publishing house, NavPress. She's very happy with them and has other books with them. However, she has some books that are too Catholic for their audience. She sold "Death Panels" to TAN books and "Rachel's Contrition" to Sophia. She loves her editor at Sophia, but that editor doesn't care for sci-fi, which is what "Death Panels" is. By the same token, TAN is not ready to take a plunge into publishing Women's Contemporary Fiction, which is what "Rachel's Contrition" is.
So writing conferences are a great place to get to know editors and what they want. As for pitching: it's a nice way to skip the slush pile!
3. Meeting like-minded writers. Writing doesn't have to be solitary, of course, and the Internet has brought us together. However, there's a lot to be said for personal communication. My roomies at the Catholic Writers' Conference Live and I stayed up until 2 am talking, laughing and sharing some pretty deep stuff. We'd only met briefly online, but we are best friends now. We went out with others to dinner and such, and I haven't laughed so hard and so often in a year.
4. It's a chance to stretch your writing wings. I'll give you an example from my Idea Generation workshop from CWCL. Rather than a lot of instruction, I answered a couple of questions, then gave everyone exercises: I presented a character, setting and conflict/object from a published book and told them to make a story synopsis, novel outline, or article query from it. Afterward, I told them the book. One woman came in believing she didn't know how to flesh out ideas, but wrote an outline for a novel and a short story synopsis--in 15 minutes each! Another writer, a novelist, discovered a talent for flash fiction. All of them left with writing they could polish and sell.
5. It's a chance to promote yourself. Writers are readers. I already have several reviews written and more books in the pile to review as a result of CWCL. Several folks I know came away with writing jobs, interviews on radio, and promises of reviews from others.
Incidentally, there's very little writing going on at a writers' conference. Writer's workshops, like Clarion, are different. Those are intense courses taught (hopefully) by experienced writers or editors and are geared specifically toward teaching you to write and write so that you get published.