When you want to spend the coins or sign a message. The password only protects the private keys, not all information about the wallet. IIRC Multibit HD is the only wallet where everything is encrypted.
http://docs.electrum.org/en/latest/faq.html#why-can-i-open-the-wallet-without-entering-my-password
"This is done in order to minimize the amount of time during which sensitive information is unencrypted in your computer’s memory."
don't leave your wallet open and remove the desktop shortcut so if a random user walked up to your screen he can't just open the wallet or see the balance & addresses.
also as shorena said set a log in password and put your computer in standby or screensaver which requires sign in.