It's a feature from Android phones, if the device is locked and you accidently press the power button to activate the device then you could start applications without wanting too.
This would also of course waste battery...
There's also the other screen locks of course, available on both Android phones and tablets like the PIN, Password or Face unlock. Obviously these protect you the user from unauthorized access to your device, personally I have sensitive information on both my N7 and my Android phone that I wouldn't want somebody to access whether they stole my device, I lost my device or just some nosey sod if I stupidly left it lying around.
Hope that helps you understand more.