Tying the rope to the crab like this is to cheat on weight. The reason is completely different.

This answer is: If you don’t tie a wet rope, the crab will easily suffer from dry leg joints and die very quickly. This moist rope, if mixed with salt water, can help crabs survive for 4-5 days. Besides, in addition to moisturizing, the rope also helps prevent crabs from breaking their legs and claws during transportation.