The surge protection devices employed are usually MOVs. These will conduct when the voltage across their terminals exceeds some rated voltage. This effectively shorts out the spike before it appears at the equipment. MOVs have a limited life, not in terms of months or years, but in terms of the amount of energy that they can absorb over their lifetime. You can independently research all about how they work and why they have limited life, but I'll just point out that as they die, their conducting voltage becomes less and less, until finally, they begin to conduct at line voltage. Not fully conduct, not enough to trip a circuit breaker, but just a little and plenty enough to get hot enough to start a fire. You can imagine how bad that would be if there was not a thermal fuse in there to break the circuit and keep your house from burning down.