The concierge performs the same services that he would in a hotel. Mostly making reservations, coordinating activities away from the ship and generally finding out things and answering questions. They take great pride in never being stumped.
A butler makes your dreams come true. On a ship he will probably be responsible for more than one suite, but you will never know it. They will unpack, press, serve you in your suite, keep the nibbles and drinks coming, smooth the way to the theater or dining room, you name it. Unless you are used to being supported by the service trade, you probably won't take advantage of half of what he has to offer.
Best approach, tell him you've never had a butler and ask what he can do for you. He will appreciate your honesty and give you far better service than if you try to bluff it. He has seen it all. Insult him and he can make you wish you hadn't.
Thought it was a nice touch when the ship next to us got underway, guests with suites had their butler standing in the background as they waved goodby to us lessors.
More than you ever wanted to know.