He would expect a dowry, and you would expect him to marry your daughter, unless there were other stipulations attached in which case you should each expect those, too.

A political marriage is not a guarantee of behavior, but rather a way of supporting a relationship by creating shared interests, ie. the progeny of that marriage.

At the same time, they were public displays of a friendly relationship. The marriage would show that the noble was your friend and you his. While such a friendship might not be perfect, it would give opponents doubts about taking on both of you.

This is also practically effective. Say your daughter's husband doesn't come and help you and you're killed. How much does he not want to live with her for the rest of his life (no divorce!)?