Comment by Leynos

An agent is a way of performing an action that will generate context or a useful side effect without having to worry about the intermediate context.

People already do this serially by having a model write a plan, clearing the context, then having the same or a cheaper model action the plan. Doing so discards the intermediate context.

Sub-agents just let you do this in parallel. This works best when you have a task that needs to be done multiple times that cannot be done deterministically. For example, applying the same helper class usage in multiple places across a codebase, finding something out about multiple parts of the codebase, or testing a hypothesis in multiple places across a codebase.