I perfectly understand both methods (discs and shells) my only problem is to know which one to use.
Let's say I had a problem it would be hard fo me to know which method to use... and would both work each time?
It's more a matter of convenience. There's not a function I can think of that you couldn't weasle through with either method. A torus is a common example where shells are a little more stright forward. This is a good example where washer would be preferred.