that I started to wonder why adults would want children to believe in Santa Claus, and whether their reasons for it were actually good.

I think that lots of people have a kind of compulsion to lie to anyone they care about who is credulous, particularly children, about things that don't matter very much. I assume it's adaptive behaviour, to try to toughen up their reasoning skills on matters that aren't so important - to teach them that they can't rely on even good people to tell them stuff that is true.