The Smart stay feature is unique to certain Samsung Galaxy phones and tablets, and allows the screen to stay on for longer than your usual screen timeout if the device's front camera can see that you face is still looking at the screen.
There may be limited situations where this is useful. Android already knows to keep the screen active while you are watching a video. If you read a large amount of text, you will be regularly touching the screen or a button to scroll the text, which will keep the screen active.
This feature may be useful in rare situations where you need to keep watching the screen, but it's not for a video or something that you are scrolling regularly.
This feature works better in some situations than others. For example, if you are in a dark environment, or not holding the phone so that it is pointing at your face, it might not be able to see your face and may not work.
This setting may cause your device to consume more battery power due to its use of the camera.