When you create a simple wallet, the NanoWallet will create a yourwallet.wlt file which contains your private key. The file is encrypted with your password. If you want to import the wallet to another computer, you need the .wlt file AND the password.
Simple wallets are more convenient than other wallet types because it's easy to import a simple wallet to a new computer just by adding the .wlt file to the new installation. Use a simple wallet if you are just getting started or if you want a wallet that's a little easier to transfer to new devices.