I don't know. Maybe guides give some folks more confidence. (There is also another, cheaper guide by "Katz Moses"). Anyway, as mentioned above, my real problem with hand cut dovetails, was not the angled cut, as much as waste removal and cutting the bottom line. I rarely got a nice, leveled line for all the tails. And of course, no guide helps with that.
Another difficulty I encountered, was making accurate markings with the pencil. The saw-blade kerf, as well as pencil width, were my obstacles when cutting nice looking dovetails.