Metals sitting in salt water and subjected to voltages passing through them will corrode. Since zinc is less noble it gives itself up before bronze, stainless, etc. thereby protecting the more valuable metal. If the zinc wears away and is not replaced then the metals in the prop and or prop shaft can begin to wear away, permanently damaging that item. You can look the list of metals and their nobility up in most boat electric books. You'll also find a more detailed explanation than mine.
There are a number of things that cause zincs to wear away. The salinity level of the water (the saltier the quicker they wear), improperly wired dock power, a neighbors power cord hanging in the water, a neighbors boat improperly wired, your own boat improperly wired (usually a result of a poorly installed modification involving electric connections) to name the most common. (the improper wiring allows stray voltages to leak into the water enabling the corrosion)
Zincs are easy to change. If the boat is out of the water alls it takes is a screw driver and a few minutes. If it's in the water then it takes a screw driver and either scuba gear or really big lungs
For added protection some folks place an additional zinc on the shaft behind the prop.
FWIW I'm on my 4th Benny and none of them have eaten zincs out of the ordinary. One exception was our last boat that sat in the same slip for 7 years. Zincs were fine all but the last year when we had a semi derilict boat next to us. Remember my comment on your neighbors boat wiring?
I hope this helps. Perhaps others will provide additional info on the subject.