A reseed host is required to create connections and onboard a new router to the network. Reseed servers can observe if a new router has bootstrapped from them, but nothing else about traffic on the #I2P network. The more reseed hosts the network has, the more resilient the network becomes.
The project is always looking for new people to provide reseed services for the core software.
Docs:
https://geti2p.net/en/docs/reseed
https://geti2p.net/en/get-involved/guides/reseed