In contract bridge, what are bonus points for?
When one side has won two games, all points (both above and below the line) are totaled, and the side with more points wins, even if they didn't win the rubber. This is where the bonus points come into play: the points for undertricks, overtricks, slams, rubber, etc. all count toward the final score.

The American Contract Bridge League explains it the same way, which I think can be kind of confusing: they talk about "winning the rubber" and "winning" without making a clear distinction. It may help to think about it simply as playing for points. Getting the rubber bonus is very helpful, but total score is what you want.