If you have a better manabase then it will generally make your deck run better. A set of shocks, fetches, and duals or fastlands will pretty much guarantee that you get the colors you want when you want/need them. Running fetches and shocks also allows for maindeck silver bullet options. For example, say you're running jund (RBG) and you want to have a single white card like ajani vengeant or maybe to hardcast lingering souls, you can run marsh flats (fetch swamp or plains) and a godless shrine or sacred foundry to hit white if you need it. The theory being that the fetches are like an extra 4 copies of whatever land you want it to grab. That, and the fact that they also thin out your deck, causing you to draw into less land and more business, is why they're so expensive.