Why would stores sell them?
There is one MAJOR reason why stores sell extended warranties -- They are highly profitable.
Although it can happen any time, electronic equipment usually fails early in its life (while still covered by the original warranty) or it fails after an extended time (i.e. AFTER the extended warranty expires).
Usually, i find it better to save the money. Instead of buying multiple warranties, I consolidate the money saved to replace or repair items that fail after the original warranty. Usually the replacement item costs less or has more features or both when compared to the original.
Why buy a 5 year warranty on a computer? You will probably replace it before then anyway!