With a scope, you simply have to line up your crosshairs (reticle) with your target. It's much easier to learn to shoot with a scope than iron sights, and since most rifle scopes also magnify, your target appears closer, and therefore easier to see, enabling you to place a more precise shot on your target.
The average deer rifle used to wear a 3-9 scope, and for good reason. Three power is low enough, with enough light transmission and field of view for close shots in most applications, and nine power gives you plenty of magnification for longer shots.
40 to 44mm is pretty standard on a medium variable rifle scope, for large objective lenses will only transmit more useable light than smaller ones if they are set at their highest power in the dimmest conditions.