We have a bunch of instances hosting basically the same images multiple times. Using IPFS we could increase the availability of images for users with almost no cost to instances. I have heard rumors that pict-rs can’t be easily scaled up due file system based storage and would need a rewrite.
I think Fediverse is major thing where using IPFS on the decentralized network makes sense.
i don’t actually think lemmy uses pict-rs’s caching functionality. afaik it uploads remote images just like it does local ones. that’s for if you want to use it stand-alone as the imgur clone it was intended to be (and not what lemmy shoehorned it into)