It goes without saying that every game in any sport becomes magnified in the playoffs. There is less margin for error, and a couple of bad performances can end a team's championship dreams and send it into the offseason.
Game 1 is a tone-setter. In a seven-game series, it allows the most forgiveness in general because the losing team still has time to make up ground. But historically the Game 1 winner in an NBA best-of-seven series is a good indicator of who will win the series.
Per the site Who Wins (an excellent resource for a lot of major sports playoff history), NBA teams that win Game 1 go on to win the series 75.