Asphalt and chipseal is the waterproof surface to keep the road's underlying gravel layers dry so they retain strength. If water gets into these gravel layers they become weak and slumping occurs. When this happens the protective asphalt or chipseal layer cracks and potholes quickly form.
Sometimes we fix a pothole, then another one appears. This is because water has already seeped further under the road surface, weakening the gravel layer and creating new potholes a few weeks later.