They're not necessary to completing the game but have a few interesting features. They serve as a time telling device, if by chance you need to know the exact time, by striking with your sword.
The more useful feature requires the mask of truth from the mask trading subquest. Examining one of these while wearing the mask will usually provide you with some trivia about the game, which can range from useful advice like tapping B to swim faster to background information about Hyrule like how Gerudos often come to Hyrule Market to look for boyfriends.
You can also do various other pointless things like bombing them to send them into orbit or burning them to make them spin.