Why? Because I'm interested by the variety of post formats (link, image, text, quote), the ability to repost stuff (so vital in twitter), and the ease of aggregation.
Eventually I want to have these things fed from Silver Insanity... Maybe even turn Silver into a community site instead of just my personal playground. But it looks like Tumblr actually has several of the things that I'd try to build into my own little blogging playground, so I'm giving it a shot. And, if nothing else, I can eventually just feed Tumblr from my own site.