Some of them have made a good amount of money through advertising (e.g. Imageshack). It seems like the market is probably a little saturated by now, so unless you've got some killer feature or a base of users to market to (Imageshack started off on the Something Awful forums) it's probably not worth it.

Any reason from "just for shits and giggles" to "getting credibility in a community" (like HN).

I run a url shortener - it costs me time and money that I will never get back directly. Hopefully it gives me some credibility in the community but other than that it is just a fun side project.