I basically do 3 things with daughters:
1) Marry them off for alliances. Here I'm looking for a ruler or heir, who's powerful enough to help me (and either we're both independent, or in the same realm, or he's independent, and I'm planning on declaring war on my leige), but not so powerful that the claims will come back to bite me. Never do this with your liege, or anyone who might become your liege or whose son might become your liege. The child will have a free pass to revoke your titles.
2) Marry them off for prestige. Here I look for a prince of high-prestige dynasty and betroth my daughter to him.
3) Keep them around for their traits/skills. If I have a genius or strong daughter, then I find her a non-ruler husband with excellent traits (the ideal son-in-law is a lustful strong genius with a 4-star education). The goal is a matrilinear marriage, but if need be, I'll accept a normal marriage and I invite my daughter back to court. Then I raise their children to become council members/vassals. This obviously works best if the daughter is far enough down in the order of succession that she isn't a pretender. A genius elusive shadow pretender is not someone you want at court.
1) Marry them off for alliances. Here I'm looking for a ruler or heir, who's powerful enough to help me (and either we're both independent, or in the same realm, or he's independent, and I'm planning on declaring war on my leige), but not so powerful that the claims will come back to bite me. Never do this with your liege, or anyone who might become your liege or whose son might become your liege. The child will have a free pass to revoke your titles.
2) Marry them off for prestige. Here I look for a prince of high-prestige dynasty and betroth my daughter to him.
3) Keep them around for their traits/skills. If I have a genius or strong daughter, then I find her a non-ruler husband with excellent traits (the ideal son-in-law is a lustful strong genius with a 4-star education). The goal is a matrilinear marriage, but if need be, I'll accept a normal marriage and I invite my daughter back to court. Then I raise their children to become council members/vassals. This obviously works best if the daughter is far enough down in the order of succession that she isn't a pretender. A genius elusive shadow pretender is not someone you want at court.