endurance has largely played a factor in the current championship game already. the game on friday lasted about 8 full hours, and they had games on saturday then sunday. after nepo played through that slog on friday and lost, he was pretty clearly gassed. the saturday game was a simple draw, then he tilted and kinda embarrassingly blundered the game away on sunday.
the reason it matters is because between games, they will often go into deep prep and make adjustments, look far down potential lines, long memorization processes, etc. but after playing through that kind of match, you dont really have the energy to make strong preparation. its part of why the draw on saturday came so easily. nepo didnt really have the time/energy to prep for his game with the white pieces (common tactic at that level of play is to push for wins with white, hold for draw with black).