Open valley flashings are seen on slate, shingle, and tile roofs. If installed correctly they can divert water into gutter systems and prevent debris build up in valley area. But they do wear out over time and need to be replaced.
A key component to an open valley is the installation of the underlayment. If the underlayment isn't installed under the existing underlayment correctly your second line of defense has been compromised.
The final step is the installation of the copper valley flashing and reinstallation of the slate. A key step during the valley installation is making sure the valley isn't penetrated with nails and is cleated off for proper expansion and contraction. Leaving you with a maintenance & worry free area of the house.