Some animals will stop dead and back out on a snare when they feel the resistance from the support wire and back out. Loaded snares have a better chance of catching those animals. Animals that blow through a trail don't have a chance with either type.

Last season I literally caught a red fox just sniffing on a beaver snare. The snare was literally hanging off an perfectly vertical drop off that I still can't believe beaver were using. An unloaded snare wouldn't of caught her.