Spinning the rotor allows you to generate more power from the wind than what you would have gotten by putting the same energy into spinning a submerged propeller.

From what I understand it's not hugely more efficient, but something like 10-20% fuel savings. Not really equivalent to traditional sails, but also much easier to retrofit and doesn't need additional crew

No mechanical linkage. Basically just installing rotor on top of the deck and some electric connections are needed. Much simpler and much less error prone. Plus keeps all the usable space on decks below.

For the same reason you use electricity for a compressor in a heat pump. It's a lever. You extract more energy from the ambient air than you've spent on the motor.

Seems an interesting solution to whales being cut up by ship propellers