The CYA shields the chlorine from the sun, allowing more of it to stay in the pool to fight bacteria, viruses, algae, etc. However, because it makes the chlorine less aggressive, the higher the CYA level is, the higher you must maintain your baseline chlorine to make sure algae stays away. Read the "best guess chlorine chart" page that Ben directed you to--it will explain it more in-depth.

The higher level for CYA is being recommended because you have a SWCG. SWCG's use a cell to electrically generate the chlorine from the salt in the water(there are those on this forum who can explain this process chemicall--I can't, so I won't try!). That cell is designed to wear out and stop working after a certain number of hours, and will have to be replaced at that point. By having your CYA high, the chlorine stays in the pool longer, so the cell is required to run less time to maintain the needed chlorine...therefore your cell will last a lot longer if you have the recommended amount of CYA for your unit. Most of them require CYA levels around 80-100 ppm.

You are smart not to just blindly throw stuff into your pool at the whim (and profit) of the pool store, and we're here to help you understand exactly what you're asking for--what each thing does and how it affects your chemistry, so you're not adding stuff you don't need to the pool.