The fact that people who buy Pokémon Go Plus will play the game without having to stare at the phone at all time is the main idea behind it. Those who have this app can catch Pokémon ever, having to take their phone out of their pocket.
Also Play POKEMON GO without Leaving the House
A green light will flash on your Plus device, when a Pokémon is close by. All you need to do from there is to press the button to throw a Poke Ball, and on your success the device will vibrate.