Think of Lambdas like autoscaling containers. If you're operating with decent load, there's a good chance your container will stay up and be reused for multiple requests. But if load drops, your containers might be shut down so they aren't taking up resources while they're not being used. When load increases again, more instances will be spun up.

It's the same approach as autoscaling VMs, but at a function level.

If the pool is still there, it gets reused. If not, a new one is created.

It should help with overhead, but not always. There's benefits to partial solutions sometimes, if you can avoid the side effects.