BMP support can be functional for picture windows, though picture windows haven't really been used in very many scripts, though I'm sure that they were put there at some point to be some sort of 'whiteboard' feature, maybe, maybe not. They can also be used as splash screens.

Since BMP support was already there, given the general size of a BMP and how well it can be compressed to a JPG (which is practically the internet standard) with little noticeable reduction in quality, generally speaking it made a whole lot of sense to use JPGs. The support for BMP (like many other functions that are superceded) is left there as to not break as much in terms of older scripts.