Why did Bobby Fischer play the Queen's Gambit as White vs. Spassky? 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 ...