If the device is used for development and testing I don't need to put a SIM in it.
It is a useless and no-sense constraint!
You don't have to input the network operator, I leave it blank.
It wouldn't let me register until I did put it in.
Just type in some random local operator.
The reason for this is because the devices can be slightly different, cause different countries have different regulations on certain RF bands, which might make your device incompatible with a certain carrier or a country all together.
There's also OTA updates which if you have a SIM card in will push an update to your phone via the carrier, which is why there are delays in updates with certain devices and network operators (This ensures that the update works as intended on their network on every device)
If you are not putting a SIM card in it doesn't affect you at all, so as long as you can flash the updates manually you are fine. Just make sure to type in any operator which is compatible with your device.