Simply; they make it fun for everyone!
Customers need some fun around the place and oftentimes they rate your cafe according to how many fun items you have and how charged they are. You can find fun items in your decoration menu and place them in your cafe. They look pretty AND they are useful.
Each of the fun items has its own battery time and they come as fully charged. In order to benefit from them, you need to reactivate (charge) them when they stop. Reactivating fun items is completely free!