It is for your good and to keep your data safe and secure. Things You Must Do Before Handing Over Your Phone For Repair: Create a Backup: there is a high possibility that your data gets erased while repairing. Remove the SIM card: make it your habit to remove the sim card before handing your phone to a technician.
Yes, Perform a Factory Reset! If you have sensitive data on your device, then reset your phone. This will protect your data from getting stolen and you will be carefree. However, remember to back up all your data before resetting your phone.