"boots" physically keep mud and debris from sticking onto the piston shaft when directly splashed on-- its a way to keep the piston`s seal happy, and promotes long life of the shock. *Most* boots will have at the minimum 1 hole in the bottom, most have 2- some have 4, two up, two down- that keeps the condensation to a minimum, and lets water have a way to leave after you try your hand at submariene captain. They do work, but its more of a preference than a true protection method. Some geniuses have even used the boots on slipjoint driveshafts- keeps the crap out of the splines to keep rust freezeing to a minimum, and makes a handy looking do~dad.
almost all of the mud/debris will be swipped off as it enters the shock body-the top seal cleans it well. Almost all are hard steel/chromoloy/non-rust promoting metals-so corrosion really isnt a factor on vehicles used frequently-
Lastly, its an equilizer of sorts. No one likes to look at a manly 4 inch thick shock with pretty stickers and nice paint job, sitting on a thin little chrome shaft. Adding that boot makes it look equal in size.