Amongst the obvious ones, such as leading a charge from the front -and getting killed first - these fortunate chaps were also expected to add to the family gene pool by meeting lots of ladies.
The lucky (ladies) were the ones who set the prince as their target, thus ensuring the best start for their offspring, the unlucky ones were those who were compelled to marry for dynastic reasons, and expected to produce heirs.
However the dice were rolled, it all involved that oldest of co-operations, sex, and princes of yore needed to be proficient at sword-work, both in and out of bed.