How come some roofs have stones all over them?

Question

Whenever I go to the hospital I look out the window and on the landing to another roof I see the whole surface covered in rocks. Why is that?

Answer 1

I guess its because of the wind.

Answer 2

maybe it helps hold the roof down in case of strong wind?  

Answer 3

The house I grew up in had this type of roof. And it was just as a previous poster stated something that held it all together with but also added a textured look.  

Answer 4

it's a type of roofing usually found on flat roofs or commercial buildings.As for the rocks they act as a binder with the tar.