uniqueness typing

[Andre W B Furtado]

I found the following text when visiting the Clean (a functional language)
site:

"Clean is the only functional language in the world which has a special type
system, uniqueness typing. It enables to update function arguments
destructively retaining the purity of the language."

Then I have some questions:
1. Can anyone explain what does "update function arguments destructively"
mean?
2. If this is a desirable thing, why does Haskell not implement it?

Thanks,
-- Andre