Perspective rulers are a game-changer, especially if you're a comic artist or any artist that draws backgrounds. Creating detailed cityscapes and backgrounds is easier than ever with the digital perspective ruler. However, having perspective rulers at your disposal will not suddenly make you an expert in perspective if you've never studied it before. Just like a real ruler in the physical world, Perspective Rulers are a tool that can make the drawing process easier.
Having access to a tool like the Perspective Rulers does not replace knowledge and practice with drawing in perspective. If you don't know the principles of drawing with depth, then these rulers will only aid you so much. A great resource that I recommend for learning how to draw in perspective, specifically with a focus on comic books, is a book called Vanishing Point: Perspective...