Yes, it's all about discoverability. I don't normally look at my vanity metrics (as you correctly put it) but I definitely get views by just sticking stuff up there. And it's just an easy way to put presentation content up without any real effort. (Also easy embedding if I want to also put a link up on my blog.)
And conversely, if I'm looking for talks on a topic, it's a good place to search for them.
That said, Slideshare isn't a big deal to me one way or another. Especially if it looks like it's going to largely go behind a paywall, I'll just start self-hosting PDF and promote through other means.