They also mirror the media to prevent it from being taken down so it’s a very important part of keeping nostr censorship resistant
Discussion
If a post contains an HTTPS link to an image hosted on Nostr.build and the admins at Nostr.build nuke the file, or a government nukes them, or their DNS provider nukes them, or the government nukes their DNS provider, that post now has a broken HTTPS link which links to nothing whatsoever.
That is the same level of censorship resistance that Google Drive has.