Originally Posted by Rogue Wave
I think it's an insurance requirement
Insurance companies are all different, so it's worth checking your policy. Most insurers will insist that the engine is locked to the boat and that the trailer is immobilised, but not all will insist that the boat is locked to a mooring.
Of course, it may be prudent to lock it anyway, just in case. A study looking piece of chain and a decent padlock works wonders when the bad guys are looking for easy pickings.