Eli Bendersky's website People have been writing journals and diaries since writing was invented. Writing down thoughts helps you to organize them, re-think them and archive them. The last point is specifically relevant to developers diaries. I had many moments of "hey, I ran into this thing before", and it took a lot of time to recall the solution. Writing interesting things down can help me find them later, and in general it is nice to track the progress of the solution of a problem.
A weblog is special, here thoughts can be easily shared with other people... Sometimes it is very important to get an external opinion on something - otherwise how do you know you are right ?
I wanted to start a weblog for a very long time. Don't know what eventually pushed me to it, but I hope it will last :-)