Why do external batteries (a.k.a power banks) have power buttons?
To save power when not in use...

A power bank's job is to be available to charge a dead phone when needed -- so it spends most of it's life "asleep". It needs to wake up only when a device is plugged in and needs power, but USB doesn't provide a mechanical means of detecting cable insertion. The only way to detect a new device attached to the USB "power" port is to process the USB data traffic and look for the device "announcing" itself by triggering a bus enumeration. This requires that you run a USB host controller and that takes a lot of power that is pure waste.

...so we design them with a button. Basically, we just ask you, the user, to tell us when you've attached something... and thanks for letting us know! :)