Ideally, when cumming, the bottom dude tends to tighten his sphincter muscles triggering ejaculation, and possibly orgasm of the top dude.
Sure, this does not happen every time. And it probably is not even a default, but it happens and is very satisfying.
For most tops I have talked about this, the matter is more that one of a dominance in psychological terms than anything related with the physics of the actual intercourse.
There is a great degree of self-validation in the notion that another guy wants you to fuck him, and that your actions help him reach ejacualation, and hopefully, orgasm, too.
A few tops obsess about their duty to bring their bottoms to the point of no return. Not every bottom is either willing or capable of reaching that stage just because the top guy is doing a good job. Hence, to a degree, a bottom's orgasm is a desirable default but not an absolute must. The same applies to the tops, too!