Generally its the delivery crew who use the sails to get the boat to where the rich guy wants to sail from. Often the crew are given a budget, for all expenses including fuel, and so anything they save on that means they have more spending money for more vital supplies like beer, beer and more beer
These crews move the yachts around the world as "the rich guy" wants spring in the Caribbean, summer in the Med, fall/autumn in the Indian Ocean, winter around Australia. So you move the yacht to meet the guy and his family/friends for the holiday onboard at specific times.
You use the diesel engine when you're likely to miss the departure/arrival dates.
Its a fun life but badly paid, but you get to spend your time on a luxury yacht.