The autoblock is used when rappelling. If the rappeller accidentally lets go of the brake line, the autoblock will automatically grip the rope and halt the descent. The rule of thumb is to use an autoblock when the risk of falling is greater than the risk of drowning.
The autoblock makes you less likely to fall if you accidentally let go of the brake line when rappelling.
The autoblock can be annoying to work with and increase the time getting off rope. It can also create a false sense of security.