why do we have web agent when we have sso server

I have a big confusion regarding sso architecture. When we have our sso(OpenAM) then why do we have agent like Apache tomcat agent 3.x.x?

Am aware of the usage of the web agent and sso work but why separate things?

Is this possible to have separate tomcat instance for web agent as we have for sso(openam) server?

This doubt is not specific to Open-AM because all the sso's are having something as I have seen same architecture .