What's a conlang?
The word conlang is short for constructed language, which is a language that was created deliberately instead of naturally evolving. Programming languages aren't considered conlangs, as they're not meant for communication between people. Popular conlangs include Esperanto, Lojban, and Toki Pona.
Why would someone want to create a language?
There are many possible reasons. Some conlangs, such as Esperanto, Lingwa de Planeta, or Blissymbols, are attempts to create a universal language that everyone in the world would speak. Some others are experiments in human psychology: Lojban's predicate-logic-based grammar is intended to make its speakers think more logically, while Toki Pona's very limited vocab requires speakers to analyze their surroundings and break them down into primitive components. Others still are created as part of worldbuilding, meant to be spoken by fictional peoples, like Avatar's Na'vi or Star Trek's Klingon. Last but not least, many people treat conlanging as an art form unto itself, not needing a higher justification; examples can be found in communities such as ConWorkShop or /r/conlangs.