Got Another Example For You
Driving along I lost my alternator and had to be towed to a dealer. (Admittedly I've never heard of anyone proactively replacing their alternator) The dealer price is about 556 and then throwing in the flat labor rate and tax, I got handed a nine hundred dollar bill from my dealer. Let's not even get into the fact that I had to wait three hours for a tow, or ride about fifty miles in a tow truck and get a friend to pick me up from the dealer.
If instead I had proactively replaced this alternator, I could get the same brand re-manufactured unit (same as what I got from dealer) for about 250.
So when I learn all about which parts most likely strand people when they fail by extensively reading posts, I take the opportunity to increase the odds in my favor by buying the parts at great discounts and putting them in free with my own labor.
You've also got the economy of scale when everything in an area is exposed, replacing many parts takes advantage of that. The free labor is great and many parts are comparatively reasonably priced.