This is a lead often used by carp anglers. This lead and clip set up is designed to allow a lot of movement between the lead and the clip. It also allows the lead to sink into soft silt, effectively increasing the weight of the lead on the take from the fish. This can help you get good hook holds because the extra weight helps pull the hook home.
However, if this set up is fished on bottoms of gravel or thin silt the lead will not sink into the bottom material. In this case the weight of the lead is not getting to the point of the hook quick enough because of the excessive amount of movement between the lead and the clip. This can lead to the carp dropping the bait before the hook is set home.