One of the main advantages is that they are small and easily reloadable. They are also relatively easy to handle. The sling allows you to be able to get under ledges and tight spaces easier as you do not have a few feet of gun in front of you. It also seems that fish are not as frightened by the sling as it is not as imposing as a gun. That being said, if you are fishing with the sling and spear you are sort of limiting yourself to small/medium size fish. That is not to say that people have not been able to bag large fish, but they usually come out looking like pin cushions. The sling and spear also have a shorter range than a speargun and there is no string attached to the spear.