I'm probably NOT the guy to be answering this, but here goes anyway.
Theoretically, the butler will book your shore excursions, make reservations. Also, he will bring fresh fruit (of your choice) in the morning and hors d'oeuvres in the evening. That's what he's supposed to do.
My experience was that our butler did none of these things. Rather, it was passed on to the cabin steward. I know my experience here is probably singular as others have raved about the butler service in the mini-suites. Alas, this wasn't the case with me.
One is supposed to tip the butler per day as one would with the steward or waiter, etc.
I'm going back on the Grand in January and have again booked a mini suite. I'm hoping for the best.
It's truly a wonderful ship and the butler service was the only drawback. You'll have a great time.