If you have a large dynasty, and only daughters, then it might be advantageous to let your eldest inherit and as Tempest said marry her to a cousin of your dynasty. Might also be good opportunity to reunite some family land (as was done historically). The only other way for female rulers to pass along the family land intact is to have a bastard by event and hope too that he survives (always he-bastards in CK1) and can get a legitimization event before you die.
And it is fun to marry the heiress of an old duke or king. The difference between the modded female inheritance and the vanilla semisalic law is that when your father-in-law dies, your wife rules in her own right rather than your (probably infant) son inheriting. But it is a waiting game from the wedding date, hoping that the old guy has no more legitimate sons, that he does not lose his titles in a war, that something else awful does not transpire. Remember, too, that the two closest heirs inherit claims on all the titles, so even if your infant brother-in-law inherits, then your wife and eldest child still inherits claims. Problem is that a baby has problems ruling a three or four province demesne and gives away titles to whoever is willing to play peekaboo with him.
... or hope they die before their father, and do not sire sons themselves.
