Amateur Historian here but i think it was because the buried person were supposed to use the wealth in the after life. In Greek mythology and Roman you had to put a coin on the person so the person could pay the boat fare that Charon had in order to cross the river Styx. In the Asatru (Norse paganism) men were buried with some of their wealth, their weapons and armor. How much was buried was dependent on the social standing of the person. This was again supposed to help the buried person in their next life be it Vallhall, Freyas field or Hel.