Testing would be one use case (I just used this exact feature to grab a screenshot of the warning message that one gets if they encrypt a previously unencrypted wallet file)... It might also be useful for users who have and regularly use multiple wallets and don't want to accidentally use the wrong wallet when generating a receive address, or accidentally send from the wrong wallet...

Another use case I could think of would be a "holding" or "savings" wallet, that you don't often need to use... but may need to open occasionally to send/receive.

By "closing" or unloading a wallet that you don't want to use, you lessen the odds of using the wrong the wallet/addresses etc.