[hahn-kev]

Except lots of applications need to be able to forget things, similar to how things can be destroyed in the real world

[fnordsensei]

Forgetting what you had for lunch last Friday is not necessarily the same as changing what you had for lunch last Friday.

You could, for example, throw away the encryption key for that fact. What you had for lunch is now inaccessible, but the fact remains unchanged.