This is quite a common site and method that do it yourselfers use for building garden ponds and waterfalls. The main reasons are that flagstone is very easy to work with, easy to transport, and is readily available in most areas.
Another reson is that flagstone is very easy to create a level spillway with. The spillway for the waterfall must be level for the water to flow evenly over the top. If it's not level, the water will run to one side or the other and not flow in a steady stream that looks natural. Also, there is usually little need for ant type of mortar or adhesive except for on the waterfall itself. The pieces of rock can simply be laid out to cover the edge of the pond.
While this isn't necessarily the best way to make a natural looking pond and waterfall that fits the elements of the surrounding garden and landscape, again, it's one of the easiest methods for do it yourselfers and home owners. It's rare and unlikely to find this type of flagstone rock in most folks backyards and landscapes. Still, with a little work and placement, most anybody can create a functional water feature that will add sound and motion to the garden.