Much like you would not send an plumber to fix a computer, and agency tries to use the agent best suited to the job. Many systems referred to as agents are really agencies but for most uses people don't really need to know which they are using. This can also be used for intelligent mail and news readers. This means beginner users don't have to read man pages, but can still probe the system for information help different users collaborate acts as a go between for processes. Users can let their agents negotiate for things like meeting times.
Most agents are Autonomous agents - agents modeled on the idea of an assistant.
They are not an interface between the computer and the user, they cooperate with the user. Agents can initiate communication, monitor events, and perform tasks. Mail readers they can be set to filter out, prioritize and forward messages. Can get problems of mail being forwarded through several users and then deleted without ever being read. Also important information may be missed because that person usually has nothing important to say.