c. some random unlanded noble has a weak claim and you matrilinerally marry them to your daughter

c. to replace humanity with horsepeople

Creating the most inbred monarch you possible can.

c) as an invite to court button that works on people you couldn't otherwise invite to your court

Marriage is great for getting claims into your court so you can land and press them when the opportunity arises, even if that opportunity comes a generation down the line, for getting good tutors into your court especially if you want to change religion or culture and for inviting and retaining good councillors and commanders.

d) murder

Marriage is also an excellent tool to have in your toolbox if you need someone dead, marrying off high intrigue people who like you and have cultural negatives to everyone in the court you're sending them to basically guarantees you another good plotter against everyone in that court, and even just plaguebombing, marrying off sick people to a royal court (difficult due to the penalties but very doable with a slight status mismatch) can absolutely decimate a court you need people to start dying in.

e) religion/culture bombing

If you marry enough people of a certain religion or culture into a court it eventually tends to stick, especially if it's combined with judicious assasinations, and it can be an absolute godsend to suddenly have the Mongol Empire flip to your religion or a kingdom suddenly implode into revolts and get targetted by holy wars because their head of state is of a different religous group to everyone else in the region now.