It's to avoid overloading them with disparate tasks and things to keep track of. They have a todo board to help them keep track of things that need doing but there are limits to how far you can push that.

Another reason: parallelism. The approach of using a single rolling continuously compacting context window is simple and OpenAI are good at compaction, so it works really well. But it means the agent can only do one thing at once. If I send it an email and it decides to spend an hour working on it, then it won't pay attention to any followup emails until after it's done. So having >1 enables more parallelism.

That said, I don't feel a need for more than two and honestly even that is kind of overkill for the sake of it. For 95% of the time I've been doing this, one was sufficient.

For free time there are systemd timers that wake it up on a schedule and it uses POSIX locks to mutually exclude runs from different wakeup sources. TODO board items can be either foreground or background; when there's an item with foreground priority the timers wake Codex up a lot more frequently than if there are only background items.