Curious child, your request can be answered many ways, but the best reason to use a longish enlarging lens for any format I know of, is to reduce your full frame enlargement possibilities.
For example, say you wish to make a full frame enlargement of a 135 negative that will fit easily inside a ¼ of an 8x10" piece of paper, virtually impossible with a standard lens, but eminently do-able with a 105mm enlarging lens.
Generally speaking, most enlarging lenses from my experience, are designed for perfect focus and sometimes colour correction, at a set magnification.
Student, or very cheap enlarging 135 lenses, are normally optimised for enlargements of around 4x to 6x of the negative.
Some quite incredible Apo enlarging lenses designed for the 135 format I have used are optimised for as "good as it gets" colour correction and focus around 20x or higher magnification. These are the Rodagon G range.
There is one standout enlarging lens I have used, the 90mm Apo Rodagon (N I think) this lens was always sought after by staff in the industrial lab I worked in. As long as the format could be covered by this lens, one would get unbelievable focus and colour correction in a seemless magnification from 2x through to about 16x. I would rate this actual lens, if it is in good condition as possibly one of the best enlarging lenses ever manufactured.
That said, whacking any good lens on an enlarger is only half the story, you really need to align your complete set-up.
Another important piece of information you need to assess, is the bellows length requirement of longer lenses. As in, does your enlarger allow you to extend your bellows enough to accommodate the longer focal length?
Clayne, has answered most other reasons for using a longer than usual enlarging lens.
Mick.