autocrypt it's a set of scripts that use linux hotplug/udev/cryptoapi mechanism to allow you autocrypting a harddrive what do I means by autocrypting : I means using a "rootplug" ie an identification device to activate the possibility of mounting automatically a crypted device.
You'll need such tool if typically you have some data you require to protect whith cryptography and that you want not creating the node "by hand" each time (this require root privileges) look at the following scenario :
you work into a company which data are sensitives
you don't want to encrypt your home using pam crypt because
you don't want to have to setup your workspace each time you come back.
So you dan't want to logout, using a screensaver to prevent use of your computer. But while you're loged the data are accessible.... a malicious hacker can login to you box via network and have access to your data....
You can manually create the cryting device mounting and umounting them at demand but this means you have root privilege you can use a special identification device, a "key" that do it for you.... that's autocrypt this solution use the widely available usbkeys/drives to create the unencrypted devices when the key is pluged and to remove it safely when the device is removed.