This is to make the document easier to find. No one sets out with a goal to set up an agent (unless they are told to). They want to monitor an application. Coming to this doc will give them a list of applications they can monitor and ways they can collect the data from the application. (Direct, SNMP Extend, or Agent)
There needs to be a lot of expansion here, but this is a start. I'd like to see most applications support 2-3 ways of collecting data.