The iPad is a fantastic, big screen devices with a great web browser and amazing games, and anyone who sees it is likely going to want to borrow it to use or just to try out. If you need to lend your iPad, however, all your personal content, messages, email, etc. get lent out with it. And that cause problems because unlike traditional computers, the iPad doesn't support multiple accounts, or even a restricted "Guest User" mode.
But with iOS 6, you can use the brand new Guided Access mode (or single-app mode) to simulate it.