Suppose you're down a couple break points in a set, so you have no reasonable chance of winning it. You are serving in what would be the sixth (and set) game for your opponent.
If you win the game, your opponent will have to serve for the sixth game, and you get to go onto to the next set with the serve. If you lose, your opponent serves first in the next set.
Yet, I've seen top players even at the grand slam events tacitly concede that sixth game, while on serve. Apparently they want to get to the next set. But why would they do this? If the server has the advantage, why not try to take advantage of this for a (relatively) easy win?
Conversely, if you're down 5-1, with the opponent on serve, might it make sense to concede the game on the opponent's serve to get to the next set with you on serve?