Think of dragons as less giant lizards with wings and more like Godzilla. In the toho films godzilla feeds off of radiation and and actively lairs in places that spew it. Dragons are similar they hoard gold and gems not because of its inherrant value as precious stones and metals, but because gold is a fantastic conductor of arcane energy and stones and gems store it. So when a dragon chooses a lair they pick spots rich in magic like leylines or junctions and build up a hoard and use thier innate ability as a feeder of magic and create a mystic sump that they use to drain magic to feed. They sleep to restore thier arcane reservoir and force healing and other biological functions.