IF you have a shell mastercard and use it at Shell, you get a 5% discount. If you use your mastercard anywhere else, 1% goes towards free shell gas.
Different locations also mean different prices: some gas stations are easier to enter and leave. Others have better gas, but I've never noticed that difference.
It makes a BIG difference in high performance cars. I lose 3 - 5 miles per gallon on cheaper (same octane) gas.
Since their gas is typically 5-10% higher than other brands, what exactly are you saving?