Thats the reaction I often get when I tell people about this hobby of mine. Languages have fascinated me from earliest childhood. I think about them, collect them, create them. And there are ways in which this helps me write better books. Primarily, fashioning a language helps me brainstorm, makes me think things through and ask detailed questions.
Semantic categories are important because they shape thinking and perception.
Navajo, for instance, makes no linguistic distinction between the colors English divides into blue and green. Some cultures only recognize two or three colors, others hundreds. In short, by creating a language, I map all of the most important features of the society I create into a vocabulary, and I make them real to myself, at least long enough to write a few stories about them. In the end, of the hundreds of words and grammatical rules I create, only a few actually get used, and then mostly as names. I try to pepper the story with just enough of these terms to impart a feel of authenticity without daunting the reader with too many italics.