Registering a device is an additional security measure to prevent unauthorized persons from accessing your account. Customers should only register personal devices, not shared. If someone registers their device, they are telling us that they want us to remember that device when they login in the future.  Their device will be recognized and they will not have to request a verification code to login, they can just enter their username and password.  This is browser specific so if, for instance, they registered from their computer in Google and registered their device, then if they logged in from Edge their computer would not be recognized as having been previously registered and they would have to get a verification code to proceed.