What is the main function of a boats safety chains?

A boats safety chain is used to prevent accidents such as a boat slipping from its trailer. This can occur when the ratchet mechanism (used to act as a stopper on the winding crank) breaks. I've seen this happen once at my boatclub. A guy tried to put his boat down the ramp and didn't have his safety chain connected and his boat just slid off the trailer and onto the concrete ramp.