If you have coil springs it's very important to keep them well oiled. Anything is better than nothing. Without lube, especially in humid climates, the coils can rust. This can prevent the coils from uniformly sliding against each other when they are loading and unloading tension. This will in turn cause one section of the spring to load up with more tension than the rest. This can result in premature spring failure.
By keeping the spring clean and wet with oil, grease, silicone lubricant, or any combination of the above, the individual coils can slide freely against each other, and distribute the load on the spring evenly throughout it's entire length. (Or evenly through both springs on a dual spring system). If you ever hear your coil garage door springs making noises as the door is opening or closing, it's all but a sure bet the springs are dry, and in need of lubrication.
I've had coil garage door springs let go, and when they do it sounds like a 12 gauge shotgun going off. They will always break when they're closed. That's when they're under the greatest tension.