This lets you a) run things in parallel if you want, but also b) keep the main agent's context clean, and therefore run much larger, longer running tasks without the "race against the context clock" issue.
I first experimented with allowing the main agent have a "conversation" with sub-agents. For example, I created a database of messages between the main agent and the sub-agents, and allowed both append to it. This kinda worked for a few messages but kept getting stuck on mid-tier models, such as GPT-5 mini.
But from my understanding, their implementation is also similar to the stateless functions I described. (happy to be proven wrong). Sub agents don't communicate back much aside from the final result, and they don't have a conversation history.
The live updates you see are mostly the application layer updating the UI which initially confused me.