I'm not much of a list-maker, but this has always seemed like a good thing to have: a reminder that there are more important things than watching television, and probably a limited time in which to do them.
There are various things in my head that I plan to do before I die -- most sooner rather than later. Sometimes they come out as a surprise when I act on them -- hiking the Inca Trail, for example. My 43 Things page starts to enumerate them as I think of them. That's part of the rationale. Most of these are things I could act on but don't, and I have no intention of using the list to generate guilt. Another reason is that it is a fun procrastination tool, they've got a fun idea user-interface-wise. This is my step into the next generation of collaboration tools -- I've been building a lot of Wikis lately, and loving them, but they are oh so 2003. And my livejournal? Ha. Dinosaur. Memestreams is almost next-gen -- could be if they add tagging.