"It shows what Blackwell’s called “the exacting nature” of Tolkien’s creative vision: he corrects place names, provides extra ones, and gives Baynes a host of suggestions about the map’s various flora and fauna." You've got to respect the man for his attention to detail. To the outsider it might seem as if it were near insanity levels (fully creating multiple language families for instance https://en.wikipedia.org/wi…
I remember reading in one of the introductions that he enjoyed working on the languages for their own sake, and would work on that if he didn't feel like working on the story. It's not as though everything he did was at this level of detail -- architecture, technology, biology, it all varies in how much work he put into filling it out. But the languages were a passion for him.
I don't think it's so much that he created the languages in service to the world and the story as that it's a core piece of the expression. I think of the series as a work of hard linguistic fiction (by analogy with hard science fiction), and seen in that light, written by a professional in the field, it really doesn't seem crazy at all that they're as developed as they are.