Other than for sport, the purpose of a 30 (or 33, or whatever) round magazine is to be able to fire 30 rounds without reloading. This is likely to be either for suppressive fire or to engage multiple targets.
This is true without regard to the moral/legal context of the encounter.
A 30 round mag can be a powerful tool in the hand of a good person. And it can be an equally powerful tool in the hand of a bad person.
Some people can't imagine owning a gun. Some people can't imagine owning a gun and using it for self-defense. Some people can't imagine using a gun for self-defense against more than one assailant, or outside of their bedroom/home. Some people can't imagine not carrying two guns, or three. Some people can't imagine walking around without one or two guns AND and an edged weapon AND pepper spray AND one or two spare magazines.
We all fall somewhere in that spectrum - if you ask the people in one direction about your choice, they think you're a paranoid loon because you're more concerned about firearms and security than they are. If you ask the people in the other direction, they think you're a carefree idiot who obviously has no regard for your own safety or that of your loved ones, or you'd be as prepared as they are.
This is true no matter who you are - there's always going to be some guy who's less "tactical", and some guy who's more "tactical", and they'll probably both have arguments for why your choice is wrong, just coming from different directions.
So I'm not going to criticize some other guy's belief that a 33 round Glock magazine is important to his security. As it happens, I don't currently have a place in my plans and preparations for any magazines like that; but maybe the tactical scenarios I consider when thinking about defense are inadequate, or at least inadequate to describe risks that the other guy faces.
It's taken us awhile to dig out of the hole that was dug in 1986 with the assault weapons ban, where we ended up designating some guns as "too scary" which effectively meant that large-capacity magazines got really expensive. I paid close to $100 each for high-cap Glock mags in the weeks prior to California's 1/1/2000 cutoff. Let's try not to do that again - I'm not going to agree, literally or symbolically, with taking away guns or components that others find helpful, even if they're not personally interesting or relevant to me. I hope that I can expect the same level of cooperation and discretion from other gun owners.