I2p is a closed network where everyone participates and using cryptography packets are encrypted so so only the destination can reason them. All packets are sent through short lived tunnels which are connections of various nodes in the network that change at every so often.

You traffic never hits the main internet. Its a full darknet with a torrent network system , email, websites, private encrypted messaging and more built on top of it.

So they are all encrypted. Nobody knows the IP of anyone else

I think the only use-case maybe not covered is finding torrents that aren't uploaded to the I2P tracker yet. They need to be cross-seeded from people who originally uploaded them or from people downloading them from the clearnet to be available.

An example of this is anime. There is significantly less anime available on i2p currently than there are on sites like nyaa.si. People can cross-seed these torrents to make them available on i2p. I do that, for instance. But other than that I can't think of any other use case that isn't covered.