It's not so much being unplugged as it is sensing whether the mains power is down. If it is, they stop backfeeding immediately, just like hardwired solar inverters do.

If this weren't done, there would be risk of electricution to anyone working on wiring in the house or on the utility distribution network.

They have to sync up and feed power in phase with the grid when its up as well, in the case of net metering setups.

It's also important for the male plug prongs to not be live when unplugged.

I commented a little too quickly and left the nuance out. Your answer is much better!