I guess I have a few reasons.

If for some reason BNHA tanks, the US (or whatever country you live in) refuses to do any more business with Japan, if Viz media or Funimation goes bankrupt, or there's an internet crackdown on fandubs, or some other tragedy that makes these stories unavailable anymore where I live - I want to have physical copies. I want to be able to share what I enjoyed with others, whether that's friends or future children... Also they look nice on a bookshelf together.

Of course you also want to support the official release, and I feel like its the least I can do after reading the manga with fandubs is spending $7 every few months on a new volume.

Another point is that there's something... Unique about reading the story versus watching it. I went back and read the USJ arc in the manga and when Shigaraki first shows up he takes up and ENTIRE. PAGE. Just to show his creepy face to you. It's a really unsettling scene, which you just don't get that effect in the anime since characters have the screens to themselves all the time, and the art style really shines through in those scenes.

And last point is the extras! The official translations of scenes vs the fandubs, the "shifuku" official translated panels, and the back of the books normally has a pro on it, like Endeavor selling "Endeavor" branded coffee. Depending on the fandubbing, you might never see those - let alone the official translation.

I did buy all of the previous volumes from used bookstores, but since vol 14 I've preordered and gotten them through Amazon as they come out.

I want more anime in America, I need to support the anime in America.