The battery permanently powers your AED so that it is in a permanent standby mode. In standby mode, the AED performs a daily self-test. This involves both checking the technical condition of the AED as well as assessing the remaining battery capacity. This ensures that your AED is always safe and ready for use.
If the self-test detects a technical error or low battery, the AED alerts you in its status display so you can take corrective action as soon as possible. Of course, this only works if the battery is also inserted in the AED. If you do not insert the battery into the AED until an emergency is actually taking place, the AED may fail due to an undetected fault or a dead battery.
And surely, you wouldn’t want to take that risk, would you?