I always wondered. Why do people learn languages like Lojban or Esperanto when you can learn some actual languages that actual people use? Why not learn Spanish, or Hindi, or Mandarin, or Arabic, or Russian... and learn a language nobody really speaks? I don't mean it in a derogatory sense, I am merely personally interested; if I had the time and capacity to learn a new language, I would gather it's both more useful…
I just learn a bit about them, but do not learn to speak, at least not enough to have the simplest conversation. But I understand how you could get more hooked into it and learn it and spend many hours with your toy language.
From all the unuseful hobbies people have, I do not think learning artificial languages is one of the worse.