I've heard of fisherman using a gl or plastic bead in front of their plastic topwater baits. Why?

The main reason for a bead between the weight and the hook is to protect the knot. With heaver weights the knot will be hammered by the weight and it will also go into the weights hole weakening and causing a break. The smaller diameter of the beads hole stops the knot from entering it. That is the MAIN reason, but it has also evolved into not only a line saver but a noise maker as well to attract the fish.