Buy Two Shoelaces, Get One Free!

The auto parts store has a sale on shocks where you buy 4, get 1 free.  Perfect for my 5-wheeled car! 

I don't believe this makes any sense.

Me either, but apparently it does to the fine folks who run the auto parts store along my commute to work.  

I often see car-related items on sale where you can buy 3 and get the fourth for free.  It's reasonably common on things like tires and rims, where the clear need for the items comes in a multiple of four.  I believe I've even this similar deal with windshield wipers or headlight bulbs where the general need would be for an even number, but if you were to get 4 of something, you could equip two cars and get a reasonable deal.  

But this sale at the auto parts store I pass every day makes no sense.  I had to look at the sign a second time to make sure I hadn't misread it.  I did not.  The sale is for shock absorbers (shocks, for short) where you can buy 4 of them (which is all your car needs...one per wheel), and get one for free.  The free one will then sit around in your garage collecting dust or something until you get that fifth wheel installed like you've always talked about. 

Who knows?  Maybe installing a shock is a remarkably difficult procedure with a failure rate of 25%, and it makes perfect sense to buy 5 of them for when you break one and have the extra one there handy and ready to go.  Can anyone confirm this?