"what does pixelfed do that Mastodon doesn't already?"

I might be biased, but I think this is an interesting question

Mastodon does provide many photo sharing features, and so does Pixelfed

The real difference is in how Pixelfed limits feeds to photo/videos compared to Mastodon's microblogging

It's a different experience, but that doesn't mean you have to limit yourself to one or the other!