In many styles of fishing it is important to have a weight on the line so that the bait gets down to the bottom. Many fish species are found around the bottom or in mid water, so when you fish without a lead you simply don't fish at the depths where these fish swim. In other styles, however, it is important that the whole rig is on the bottom, for example the paternoster with seaweed when you fish with your beach rod. Also, when fishing for carp or with a method feeder rod, the whole rig should lie still on the bottom. You therefore need a somewhat heavier lead weight for this.
Fishing leads come in various shapes and for each discipline the ideal lead has been designed. You may only need a few grams of lead when you are fishing with a fixed rod and a float, but when you are wreck fishing at sea you might need 300 grams of lead. In general, a lead weight is used in almost every fishing discipline, some more than others.
Lead is a substance that can be harmful to nature, which is why more and more lead substitutes have come onto the market in recent years. In order to prevent the toxic lead from ending up in the underwater world, other materials are used. For example, carp weights that are made from stone. These weights are slightly larger than those made of lead, but a carp lead assembly made of stone is still very usable and certainly a suitable alternative when you think of the environment. Weights made from iron, copper and tungsten are also very popular as an alternative to lead. In dropshot fishing, for example, it is already common to use a tungsten dropshot weight. The high density of tungsten makes the weight smaller than when using a lead. This way you can fish even lighter and because tungsten is such a hard material, your bites and bottom movement will be even more direct.