Tapered dip bars are designed that way for a couple of reasons. The first reason is to make designing and manufacturing easier since the tapered design allows them to attach at one point where they come together. The second reason is more applicable to the person using the piece of equipment. The tapered design allows you to change your hand width, which changes the muscles that are targeted while doing dips. If you take a more narrow grip, that will focus more on the triceps, whereas a wider grip will hit more chest and shoulders.