Before sending in all accounts and MDM tools as well as all customer data at a device should be deleted to give our Service Team access to the device without reveal customer data.
Why Service Team has to enter the device?
For finding the reason of an defekt and to proof that there is nothing else defect at customers device it is necessary to have access to the full functionality of the devices.
If this is blocked by a PIN or restricted by a MDM software we can't do a complete check.
also after flashing the firmware, the access is not possible regarding to the Google security features of Android. If there is set an Google Account the device request after the factory reset to enter the password.
Related article: How can the Google Account on a device be deleted?