I suspect it's just another less-than-truthful advertising ploy. The premise is, if they tell you other customers bought it, there are several unfounded assumptions most people will jump to on their own:

(1) "Customers" means many people, when in truth it may have been only one person per item listed.

(2) The customer bought it recently, probably in the same shopping order, and for himself.

(3) If other people bought it, it's probably good stuff (whereas most people are sheep in their musical tastes, buying pearls and imitation paste with equal avidity. And you, likely being one of those sheep, will buy what others do because there's safety in conformance.)

(4) The items are representative of what people bought, rather than being selected mainly because the seller wants to move those items.

(5) People's buying habits are the most trustworthy testimonials (though the sampling presented to you is not truly indicative of what those habits might be.)

You may find it interesting, even occasionally useful, to peruse "customers also bought" lists, but never forget who is really in control of them--and how skewed or artificial their recommendations are.