Because people will realize that most of the current apps they use are modifying what users have said/made in the past without their permission. You probably didn't notice, but I am willing to bet every time you export your photos from a preferred server (Google/Apple/Next cloud/etc) a few of them are missing and other have been changed. Same for documents, PDFs and so on. I have lost many pictures/documents in that way.
I don't think nostr fixes that problem, because a relay can delete data if they want to as well. If we want redundancy and data integrity we can already get that by encrypting data locally and backing it up to more than one place (no nostr required).
Yep, but in nostr you have to hash every single item you publish, making checks very easy to implement, compare and reshare. You don't have to think about that before you start using the service. It's backed in.