Use sweep if someone else knows the private key.

For example:

You are buying bitcoins from someone.  You arrive with cash in your hands.  They arrive with a paper wallet that is already loaded with the amount of bitcoins that you are purchasing. You have know way of knowing if they kept a second copy of the paper wallet.  If you just import, then you and they both still have access to the bitcoins.  They could spend (steal) them before you get around to using them.  If you sweep, then they get moved to a new address with a new private key, that way they can't still access the bitcoins with the private key from the paper wallet.