I just read this article, Why you Should Learn a Language and noticed that those are reasons to learn lingua francas, ie. languages spoken by a lot of native speakers or that are routinely used by people as a 2nd language for communication.
So here is a list, mostly inspired by “Why you should learn Na’vi”
* means this is something that only or especially applies to conlangs, deadlangs, endlangs (endangered languages) Most of these items also apply to English, French, Japanese, etc.
1. You learn linguistic jargon and concepts as side effect
2. It is entertaining. It’s a hobby. Occupies free time.
3. It acts as a form of encryption (stronger for conlangs, weaker for French or Japanese)
– encrypting insults and other socially unacceptable speech seems to be a theme.
4. It is prestigious to have an unusual skill (sometimes, to some people)
5. An exercise in language acquistion, telescoping, to make subsequent language learning easier. Especially if you don’t know what language you will eventually need to learn or if the conlang is smaller than a natlang. Otherwise, you maybe should just learn your target language first.
6. An a mental exercise to make you more intelligent in other domains, like logic or essay composition
7. S-W effects– see the world differently due to different grammar and lexicon
Specific S-W effects, sense of direction, peacefulness, ability to see five minutes into the future, etc.
8. A sort-contrived common ground for making friends, the same was that sports and weather discussions are a common ground.
– also to join a community, applicable if the community already exists
– to join a likeminded community. Conlangs tend to attract people meet the advertised target audience)
9. * Makes an artisitic work more vivid (Klingon, Na’vi, Sindarin/Quenya etc)
10. To be used in pranks
11. To be used in music as a new and different sound
12. * To learn how to make a better constructed language (for all the various reasons that conlangs are created, recreation, economic, medical, psychological)
13. To work in the few movies, and other places that use conlangs/dead languages as an input into movies, games, plays etc. This isn’t cost effective enterprise though, few people will be able to create or learn conlangs fast enough to make it worth the typical few thousands of dollars conlangers get paid for conlang work.