I2P is basically just an anonymous network layer. It doesn't store files, proxy to the internet, run a web server, etc. All it does is anonymously connect standard servers and clients.
Freenet on the other hand is an anonymous content distribution system. You can't use it to connect arbitrary network services together, it's only good for storing and retrieving files. Nodes on Freenet each store encrypted pieces of data but it's computationally hard to figure out what data a particular node is storing unless you have the key to access it. Files are prioritized based on popularity so old files that no one has accessed in a while and the original host is gone can just be lost forever. It's similar in some ways to bittorrent swarms. Over time as a torrent becomes less and less popular you're likely to run into issues finding a seeder that's still around.
Freenet on the other hand is an anonymous content distribution system. You can't use it to connect arbitrary network services together, it's only good for storing and retrieving files. Nodes on Freenet each store encrypted pieces of data but it's computationally hard to figure out what data a particular node is storing unless you have the key to access it. Files are prioritized based on popularity so old files that no one has accessed in a while and the original host is gone can just be lost forever. It's similar in some ways to bittorrent swarms. Over time as a torrent becomes less and less popular you're likely to run into issues finding a seeder that's still around.