The answer is simple - to protect your data.
There is a bug that can bypass security in iOS 7. This bug allows anyone to disable Find My iPhone and bypass Activation Lock without a passcode. Disabling efforts don't always work and it's difficult to complete, but if someone does steal your iPhone, or if you lose your iPhone all your data can be compromised. The bug is irrelevant so long as your iPhone is passcode protected.
So to be secure, put a passcode on your iPhone and have a great weekend!