While the "cache all the things" media storage behavior of Mastodon is a lot of duplication and increases needed storage space, it has been handy as I'm following folks who have instance outages.

E.g. a friend's been working on their ARM-based hosted at home instance, it's been going offline randomly (I was going to boost a post of theirs this evening, only to see it's offline again), but I can still catch up on their posts - including photos - as my instance has been online meanwhile.

(I'm not against more control for server admins, but I do think "cache by default" is a good policy.  There's also privacy and security concerns around targeting exploits to certain IPs or devices.)