Basically, it gives the user the ability to decide which media provider to use in their preferred media client. This also implies a home server that the user has set up at home (as long as it is compatible with NIP96).

It helps to decentralize nostr, and also allows developers to forget about this (I have had to bother many developers to add nostrcheck.me to their app), with NIP96 they will only do the work one last time.

We will add and remove media providers as we do now with relays.

There are apps that do not want to add nostr.build, others do not want to add nostrcheck.me, this decision is up to the user, not the developers.

I see it as so important and necessary that I can't explain why it is not already being implemented everywhere

I am going to try to PR as many nostr clients as I can so that (if they accept it) they will have compatibility with NIP96.

(Sorry, I got a long answer)