We were told we needed a zinc anode to go into the skimmer for our salt pool, why?

​SACRIFICAL ZINC ANODES FOR SALT WATER POOLS
 
How it Works
The Sacrifical zinc anode protects all metal parts (heaters, lights, rails, etc.) from galvanic corrosion. Zinc anodes will sacrifice themselves and corrode before all other metals in the pool. Zinc anodes need to be replaced approximately every 4-5 years.
 
Reasons for using Zinc Anodes in chlorine-generated salt water pools
Any time you have different metals (copper, stainless steel, etc.) in a salt water pool, you create a battery. Some amount of current flows between the metals. The electrons that make up the current are supplied by one of the metals, giving up bits of itself in the form of metal ions to the pool water. This is called galvanic corrosion. Galvanic corrosion causes plaster discoloration and metal erosion. The best way to inhibit the effect of galvanic corrosion is to use a zinc anode. Zinc is a metal that gives up its metal ions faster than other metals in the pool. In other words, the zinc anode will erode instead of other metals (pool light, rails, heater, light niche, ladder, etc.). The zinc ions will not discolor the pool plaster. The zinc anode should be replaced after half of it has eroded. This takes approximately 3 years.