Why trucks transporting stone should have the stone covered with a tarp

It happened in Bratislava, Slovakia. The luck of this driver is that he had a dashboard installed that caught this incident.

A dump truck entered the highway in front of him. It appears that this truck was carrying gravel, because a stone simply flew from the dump truck directly into the windshield of the car behind it.

It can be seen on film that the stone was quite large and at the end there is an image with the car windshield.