Anything can break or fail. For example this Ruger Security Six:
Failures like this are rare, however it can happen. Story goes that Ruger used a corrosive lubricant on their barrels for a while which compromised the crush fit that they used.
So the point of the matter is that all the items you may depend on can fail you. Thus it is good to have spares. The common saying is “Two is one, one is none.” Some people take this to mean that you should have two copies of everything. For a short while, you even saw some trainers mount two flashlights on their rifle in case one stopped working.
Personally I think the better goal is to maintain effectiveness, not to have an exactly backup of everything. So for example, if the light on your long arm fails, a hand held light can get the job done. While it may not be practical for us to carry two longarms into battle, having a pistol greatly aids in the ability to keep fighting should the longarm fail.. On the CCW side of things, if you do not carry two pistols, a knife, pepper spray, kubotan or other force multiplier is far better then one gun should that gun fail.
Think like a spare donut tire in your car. The donut tire isn’t great, but is far better than having no spare. Having duplicates is a nicety, but redundancy is required.