I have used the services of the butler on four of my NCL cruises. It is a wonder perk for paying for a penthouse suite. The butler is there to get to you whatever you need from the ship. He will deliver your room service orders to you, bring you nightly treats from the hotel staff, get you DVD's to watch on your television and work as your liason for any hotel services you will need onboard.
You should meet your butler on embarkation day at your suite. He will explain his services to you. He will bring you treats in the evenings from the hotel staff. Your suite should have a DVD player. There is a list of available DVD in a book on your coffee table. The butler will get the movies you wish to see.
The best of his services is in room service. In the room is a book that has a room service menu on it. Unlike the room service menu in other cabins, the penthouse suites have more hot food items, including a hot breakfast option. There will be a form that you use to order breakfast from. You can order hot items from the menu as well as write in some that you don't see on the menu. Every morning I ordered a half a grapefruit and it was not on the menu. I also ordered Eggs Benedict several mornings. You set the time you would like the breakfast delivered and the butler will bring it to you and set it up for you on your table. With the Butler's Menu you can order your lunch or dinner from the menu from the restaurants on the ship. The butler will bring it and set it up for you. Some people have said that they have had the butler actually serve the meal in courses, but I have never had that service.
After a long day ashore, it is nice to get into the "grubby" clothes and have the meal from the diningroom delivered to you.
Remember that the tips the NCL deducts daily from your onboard account does not cover tips to the Butler or the Concierge. I tip well because I use their services a lot on my cruises.