I don’t think the book aims at being “useful” in the usual sense of the term. Neither the minimal language it builds does.

(They are probably “useful” in the dissemination of what the real essence of computation can reduce to, in practical terms.)

Not everything needs to be useful in fact: certain things can be just enjoyed in their essence, just looked at and appreciated. A bit like… art?

I am implementing my own Scheme as well. Why? I don’t know, one needs to do things that serve no apparent purpose, sometimes.