The Crook can be used to double your chances of getting saplings and apples when using it to break leaves and to obtain Silkworms. Right clicking a mob with a crook will pull it towards you. This also works with the Crook in your off-hand. Left clicking a mob with a crook will push it away from you dealing no damage.

Craft like so:

Please note that the crook doubles your CHANCES of receiving saplings and apples; it doesn't double how many you get.