They're used to remake furniture at Re-Tail. You can't remake furniture until Cyrus (the blue alpaca sleeping on the right side of the shop) wakes up/unlocks.
"The player must first sell 100,000 Bells worth of items to Reese, have owned 100 different types of furniture and 50 different items of clothing in their catalog. They must also have been in the town for over 7 days." - Other topic
"The player must first sell 100,000 Bells worth of items to Reese, have owned 100 different types of furniture and 50 different items of clothing in their catalog. They must also have been in the town for over 7 days." - Other topic