It takes a key to start up your tow vehicle and drive it away. But anyone with a tow vehicle of their own can uncouple your trailer, couple it to their truck, and drive it away. So a simple lock through the coupler can keep it from being uncoupled. And an unattended trailer can meet the same fate. The same lock can keep your coupler closed, keeping someone else's trailer ball from mating, preventing theft. But thieves are canny; they know that even if the coupler is locked onto your hitch ball, it's easy to pull the pin on the ball mount, mate the mount with their receiver while it's still locked, and steal your boat, RV, race car or whatever.