Set piece games allow you to play using a standard opening, or replay a game which got spoiled for some reason, because you can put the pieces wherever you like on the board. They're also used to generate random position pieces for a Fischer Random / Chess960 game. Always unrated, and they wear a special symbol next to the game in your MyGames list. Try out the two options on the New Games page to see how they can work.
Haha, I ask because one day, I opened up a game half way done that I didn't remember playing with an opponent I didn't recognise! I have just figured it was a night with too much wine! Today, I noticed that icon beside his name. So, I guess this explains it, eh?
I've looked at that game, and if you go to the Game History page and click on "start" the game will show at the point where the humans started making moves. He may be trying out a new opening, or re-creating a game he's just lost to figure out how he could have played better. Ask him in the game message.
Early chess in India had a set of moves played by both sides at the start of each game. The resultant positions were similar to modern King's Indian (where the name came from). But that was referred to as tabia.