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.