Some good points made earlier but there were a few more pros for Iron Sites.

They are of course quite tough. They are connected directly to the weapon so unlikely to be "banged" loose without a serious blow. They are the quickest to use on close or moving game. They are never "out of focus" and never fog up. They cost nothing, as they come with the weapon (usually). They never "bite" you when shooting from an akward position. You don't have to lose sight of the animal when bringing the weapon up either, with scopes you often have to "find" the animal again if you have any kind of magnification turned up.

I hunt both, I prefer open sites for still hunting and working in thick cover. I prefer scopes if I expect to be shooting at animals in excess of 75 yards or if I am in a fixed spot.

You definitely don't "NEED" a scope, but depending on how you like to hunt and where you hunt they may help you be more accurate.