I would not rig my boat without them they are that good and reliable. They prevent your release clip from sliding down from the tip of your outrigger.
Hal-Lock Lockable Outrigger Pulley Anglers have been using outriggers on boats in search of more and bigger fish for many decades with major improvements and innovations from the days of the bamboo outriggers- but the pulleys used with the outriggers today are the same basic design used for thousands of years. Anglers trolling with outriggers know the aggravation of the release clip working back to the boat while trolling for trophies. The most common fix for this halyard / line creep has been to squeeze the halYd together above the pulley and tie a rubber band around them or wrap them with some wire so they will not slip- (then after catching a fish or a knock down you still have to untie or unwrap the halyard to bring your clip back to the boat)- or to increase the tension on the halyard making you have to pull harder on the halyard to get the release clip in and back out.