The power button on MacBook allows you to put your Mac into sleep mode. To do this, just hold down your power button for one second and release it.
When you are away from your desk, you might leave leave your machine on the desk, it will takes some time to go to sleep mode depending on your power savings settings in your system preference. If you want to put it sleep quickly, then the power button will do.
Besides, there is a little changes on the behavior of the MacBook power button. Previously you just needed to press the button once only to go to sleep mode.
Changes of Behavior in Power Button
After the recent system update by Apple, the power button no longer works if you press once. Now you need to ‘hold and release’ to execute the ‘Sleep’.
I find it good especially when you accidentally pressed the power button while typing on the MacBook keyboard. So you won’t get into unwanted Sleep which interrupt your work there.