Butlers are great for unpacking your things and making sure you have everything you need to be comfortable.
On our Scandanavia cruise we had our Butler bring us fresh brewed Cova coffee every morning; it was great...and complimentary. We decided to have one formal dinner in our Sky Suite one night, and he dressed the table, and brought every entre' just as it was presented in the Trellis Dining room, very nice. Other than that, we really couldn't think of anything else!