I'm not desperate at all, in fact I had saved up money to fund the project myself for a couple months. The donations are because my goals in Mongrel2 changed once I realized I could actually change the lives of programmers and maybe put an end to at least a tiny bit of the language religion that seems to infect the web programming world. With a bigger goal and one that needs me to give the software away for it to succeed, I decided to ask for donations.
The donations serve two purposes: They make it so I can work on Mongrel2 for a while and get something awesome started. It also demonstrates that other people are backing what I'm doing. When you don't have a large community at the start, people will take monetary amounts donated as a sign that the project might go somewhere.
I can't say I agree with folks who judge a project's success by donation amounts or community size over technical capability, but I can understand it.