That flashing is left exposed so the roof has visible lines of metal running down the valley.
Roof valleys metal or shingle.
In addition it is important to avoid nailing the shingles through the valley metal.
These items can be fabricated in many different profiles.
With an open valley a strip of metal will be visible on the roof wherever two opposing roof planes meet.
Whether for slate shingle or metal roofing valley flashing adds detail and helps to direct water.
Roofing shingles are installed then cut and sealed where they overlap the metal channel.
For example in metal roofing systems valleys are commonly fabricated with hems but can also be fabricated without them in which case they could still be used for metal roofing or any other type of roofing.
A chalk line is then used to strike a line from the top of the valley to the bottom of the valley.
The shingles are then cut out of the valley area opening the surface of the valley lining to water run off and the environment.
The metal is usually pre painted to best complement the shingles color blend.