They are typically entered during the installation process or when activating software for the first time.

“The key to the lock that holds all other locks closed. Turn it, and every door you’ve ever unlocked slams shut forever. Your green lights go red. The toast burns. The library burns. You go back to the life you had before you opened the box.”

Deleting user accounts (e.g., Uplay) can permanently destroy the license associated with your serial key, as discussed in Steam Community forums Widgit Software For finding a lost Windows product key, Microsoft Support