It’s the Thundering Herd Problem.

See https://en.wikipedia.org/wiki/Thundering_herd_problem

In short, if it’s all at the same schedule you’ll end up with surges of requests followed by lulls. You want that evened out to reduce stress on the server end.

It's just a safe pattern that's easy to implement. If your services back-off attempts happen to be synced, for whatever reason, even if they are backing off and not slamming AWS with retries, when it comes online they might slam your backend.

It's also polite to external services but at the scale of something like AWS that's not a concern for most.

Helps distribute retries rather than having millions synchronize