Why did Bobby Fischer play the Queen's Gambit as White in many games in the 1972 World Championship match vs. Spassky, although he almost never played it in any other tournament or match game?
Spassky had an entire squad of Soviet grandmasters trying to find holes in Fischer's repertoire. So it made a lot of sense to surprise them. He also avoided the sharp King's Indian and went for the Nimzo instead.
There are other examples for this strategy. Peter Leko, a 1.e4 player, switched to 1.d4 for his match against Kramnik. Changing the black repertoire is even more common: Gelfands Gruenfeld and Sveshnikov against Anand, Kramniks Berlin against Kasparov ...
Another idea is that it was strategy at it's deepest. He waited until the most important match of his career to use this part of his opening repertoire. In "Fischer-Spassky, Reyjavik 1972" C.H.O'D Alexander writes of game six. "This game was notable for two things. First, Fischer played the Queen's Gambit for the first time in his life in a serious game; second, he played it to perfection, the game indeed casting doubt on Black's whole opening system." In the game notes he describes it as a 'sensation'. Talk about catching your opponent unawares. And is there any better strategy than that?
Fischer was noted for his attacking play derived from Kingside openings. He had to figure that Spassky (and other Soviet masters) would analyze this play.
Therefore, he played variations, particularly queenside games, where he didn't have much of a "footprint," where people couldn't prepare as well.
In essence, he was "starting from scratch, and relying on pure skill.