Reseed hosts are needed to for bootstrapping, that is, providing the initial set of I2P nodes for your I2P node to talk to. Depending on the status of your node it may need to bootstrap every now and then if many of the nodes it knows of aren't contactable.
Reseeding is done over an encrypted connection and all of the bootstrap information is signed by the reseed host you connect to, making it impossible for an unauthenticated source to provide you with false information.
The more reseed hosts that are run, the more resilient the I2P network becomes, and the harder it is to prevent users of I2P from connecting to the network.