Good for several reasons:So this is a question for all that know, obviously I don’t. My SMP is well loved and worn as a daily by the previous owner. He disc golfed, golfed, dove, worked out etc. I got the full kit with warranty cards. If I did have the watched serviced and refurbished, do the cards save on the cost?
1) Some brands won't allow a factory service without them. Some Rolex Authorized dealers will keep the card to stop a buyer from turning a quick profit on the gray market. See Thor447's response about Factory Service, sometimes it is less expensive.
2) Does add value on resale, especially if there is a warranty left.
3) Although not perfect, does help to provide provenance and verify authenticity.
4) Some people will sell the box and papers separately, keeping the watch and getting a little money on the side.