Perhaps for one or some of the same reasons many, many residents I knew did so. In some cases it was an attempt to stay in a certain geographical area, in some cases it was an attempt to associate with a particular institution, in some cases it was a lack of certainty regarding what they wanted to do so applied in more than one area, and in a lot of cases there was a lack of certainty as to whether they would get a fellowship at all so applied to something general or a fallback. I think the latter is still a fear for some residents, but probably the least realistic -- fellowship opportunities on the whole are a dime a dozen, although the "good" ones take a little more work to get into the year you want to get into them. Derm I guess is more difficult since you're competing with dermatology types. I don't recall anyone who did one "just to do one", but maybe that's a matter of perception -- what one person sees as doing a fellowship to improve competence and confidence, or to theoretically improve their long-term career opportunities even though they had a job offer prior to fellowship, is another person's "just to do one"?
Unless someone is independently wealthy or really quite strange, residents doing a fellowship for my version of "just to do one" just doesn't seem to occur. That doesn't mean I think everyone who does one actually needs to, or is going to get out of it what they hope to, though.
What the heck do you think is the purpose of this forum?