It means that nobody asked for or requested it, but they still do or state something stupid. They just do “that”, even if they don’t have to.

I think it originates and makes the most sense in someone posting a wild defensive explanation for something that wasn’t attacked to begin with, only because they want to state their opinion on something that nobody gives a shit about.

If the thing is stupid enough to be funny on it’s own, then it can easily be omitted, but it has that extra layer of the poster basically asking “why’d they wanna do that”. This indicates that the poster themselves is two steps away from whatever foolish thing is going on and only observing them for amusement.