Something that had no value when it was placed/discarded/dropped might be of considerable value now. Imagine if a discarded preparatory sketch by Leonardo Da Vince were to turn up in some Florentine cellar.
Wouldn't some grave goods fetch a decent return?
Edit: Of course, a barrow and a cairn are not the same thing, but to someone unfamiliar with the nuances of prehistoric structures, some unscrupulous scallywag might make the same mistake that I just have.