It's weird that this question keeps coming out in several cool but not that useful HN posts.

Some times it's cool to do things just because you can, maybe he was bored or he always wanted to implement a lisp dialect.

It might not be that useful, but i'm sure that the autor had a blast doing it, otherwise he wouldn't have done it.

But really the "why" is that it was fun. Also, it was my submission to the PLT Games. http://pltgames.com/competition/2012/12

Why not?

To learn.