From a forum discussion located within this awesome post about something else:
Q: Why is there always this need to play devil’s advocate? Not everything has a valid “other side”.
1) Some people believe that slapping phrases like ‘devil’s advocate’ or ‘thought experiment’ on the front of an argument is a magic escape clause if the argument is crappy or ill-thought-out. “Hey, I wasn’t saying something terrible, I was just playing devil’s advocate!”
2) Sometimes when a discussion seems to be mostly people agreeing, there are folks who find that uncomfortable and believe there must, has to be, a dissenting view thrown in, because conformity and sheeple and stuff.
3) To wit: “If only these people had my wisdom and rationality, they’d understand that they could solve this problem quite simply!”
See also this from The Toast.