There are two primary reasons things fail: Infant mortality and breakage due to normal wear and tear or abuse. Infant mortality refers to things that break early in their life due to a manufacturing defect or because something slipped thru a quality control check. Normal warranties are designed to cover Infant Mortality issues.
Extended warranties are written to cover the second area of failures but are normally written to try and give the insurance provider enough wiggle room to get out of paying. So if they can show or prove that the failure was due to abuse they probably won't pay. If they can show that the failed item worked past it's average/normal/expected lifespan and that it just "wore out" they probably won't pay.
Some folks get a certain level of comfort in buying extended warranties, I'm not one of them. If it fails during the Warranty Period then it's covered. If it fails after that I figure it's normal wear and tear or some stupid on my part and I accept responsibility. That approach has worked for me and I suspect I've saved a lot of money not buying extended warranties over the years.