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.
You write "although" as if that were a reason not to do it, when it's the opposite. Imagine if he had done otherwise ... then someone could reasonably ask "Why did Fischer play the same opening lines he had played in all his other tournaments and match games, although that allowed Spassky's team to thoroughly prepare for them?"
Fischer knew the Soviets had analyzed every move in his opening rep and he knew his only chance of winning was to change things up. He played other openings that weren't typical like Alekhine's and the Nimzo-indian.
A lot of people (especially younger) don't really understand the cold war and don't understand how important chess was to the soviets. They devoted enormous resources to beating Fischer. Im sure they had rooms with hundreds of strong players analyzing Fischer's games day and night. There was an account in Brady's book about an adjournment that kind of confirms that.
Also, there's a strategy in matches. You don't want to just play the same opening over and over. Part of that strategy for Fischer was to get the analysts focused on other openings and not what he actually wanted to play.
Also, Fischer was criticized for playing a limited opening rep and I'm sure he was answering that criticism.
And, I'm sure in preparing for his title run, he looked at Spassky's games and found some positions he thought he could exploit.