There are several types of power line issues and basically two types of protection.
SPIKES, these are high speed transient peaks in voltage. that can easily hit 1,000 volts or more. Very common when say an Air Conditioner shuts down.. SPIKE suppressors, also called surge guards do a fairly good job of clipping these using either Metal Oxcide Varisistors (MNOVs) or less often gas discharge tubes (Far less often). You have these in your house as well and always have so as you can guess. though they do damage,, , over time... It can take a LONG time.
The other thing is high/low voltage, THis is far worse,m and the nore expensive monitoring systems (These usually have an LCD display) watch the voltage and if it goes into the danger zones (Too high or low) Cut you off to protect you because OFF is safer than Danger zone.
Another advantage.. They delay re-connect.. So imagine what happened to me happens to you.. BANG, no power.. only the BANG was outside the camnpground (Car v/s utility pole, minor injuries only)
Well when power was restored ... all the air conditioners in the park tried to restart at once.. save mine.. For the first minute or so that asd oner voltage rollar coaster.. But in muy rig the little box was counting 1,2,3...... 167.169, 169.. clunk (almost 3 minutes) during this time that rollar coaster ride came to and end.. THEN I got power.. Nice, stable power.