A host cache is a program which hands out IP addresses to gnutella clients. When a gnutella client is started it needs to find some other gnutella clients in order to connect to the network. The host cache provides those IP's. The hostcache is usually the first place you connect to when launching BearShare. Once you have received some addresses from the host cache then you attempt to connect to those addresses. The only reason to communicate with the host cache after that would be if all the IP addresses you had were not working and you needed some more IP addresses to try.
This is from a tecnical FAQ for BearShare, they use what are called ultra peers to get you started, these are saved in a "host cache" as stated above. I am not sure how they could monitor this unless the "ultra peers" are giving them our IP addy's or something.