Why do you need more than 10 round mags?
My first instinct is to ask back, why do you need to ask?
Do I ask strangers why they own something, with the implication that I not only think that they should not own it but also think they should be penalized for owning it? No.
Some folks need to realize federal gun laws carry criminal felony penalties, and us "gun nuts" don't take kindly to folks calling penalties down on our heads over "shoulder things that go that-a-way" when it is really no skin off their noses (which should be up their own business, just as I keep my nose in my business).
I personally have several guns with ten round or less magazines, including the family's three designated home protection go-to guns. So I do not need more than 10 round magazines. I have single shot hunting rifles, including muzzleloaders, so I don't need for than five in a semi-auto for hunting (unless I take up feral hog hunting, magazine limits don't apply in varmint hunting).
I also have guns as collectoble for historical or design study, including "assault rifles" I shoot in vintage (used pre-1946) and modern (used 1946 or later) matches. They use standard capacity magazines of 15, 20 and 30 shot capacities, as issued or exact reproduction. Because that was what was used, and a non-issue magazine would be ahistorical for collection and not suitable for civilian marksmanship training for family eligible for volunteer military service.