What are reasons to construct a language? Why do people construct languages? For example, auxlangs are created for global (or regional) 'compatibility'. Why do people construct artlangs, altlangs, englangs, auxlangs and philangs?
I cannot speak for any other language creator, but I have done so as part of "worldbuilding" - for an alien world to have alien languages adds verisimilitude, and even if the language isn't actually heavily used, knowing how the language works and having a partial vocabulary lets me insert words from that language where there is no exact translation in English, or where the English would be a long circumlocution. As an example, one language I worked up has a word that means, literally, "a change in lighting that reveals new detail", but whose use has been extended to the metaphorical equivalent - and in one discussion in the associated story, after one character explained something to another, the second thanked the first - "That was a very useful kurìshdàm; thank you."