For a princess to appear they need to be the duaghter of the heir or king. Their quite useful while you have them. They seem to earn charm ratings feriocisly quick in diplomacy. I'm not certain of this but are the quality of the suitor's dependant on the princesses charm rating? I've also noticed that princesses in general get better suitor's then a non princess female family member.

A word of caution though. Never marry a low charm princess off. The low charm (0-3) will actually hurt the generals stats, and make him less fertile. The higher their charm the more they will improve the general when they marry them.