What Exactly is the Purpose of the Museum Lot?

You should keep one at least because it's required as a step in completing the Painter Extraordinaire aspiration.
I use to use them as a place to take dates before we got more venues. Now mostly my wanderers land there I usually put out more easels so you can paint without buying an easel (can help if your sim is poor).
I think it's nice too in general because we do have museums in real life so ... it doesn't have to be there but some people will want and use it.