Yes, both teams will play to win. But the Giants' situation still matters. For example, it could affect whether the Giants' coaches knock themselves out to win the game, or whether they spend some time scouting the next week's possible opponents, hold back a trick play or two, and so on.
For another angle, suppose the Pats are 15-0. The Giants will surely start with their starters and so will the Pats, a couple of the most banged up ones perhaps excepted.
But if the Pats go up by a couple of scores, how quickly will the Giants go deep into their reserves? That depends a lot on whether they need the win.
And if the Giants go deep into to their reserves, then the Pats can perhaps get away with resting more of their own players and preserving them from injury too.