By no means an expert, but from what I've found out:
You gain prestige. You also gain passive prestige-gain each title you own (although you can't own too many without opinion penalties) and passive renown-gain for highest title. Forming a duchy on counties for a vassal will improve their opinion of you for some time.
Getting congenital traits on your spouses increases the chance of passing them down on the children. You probably don't want the negative traits, but the positive ones are good.Other then forming alliances does it matter who you pick to have your children marry?