Often knife sharpeners for sharpening knives and knife honing for maintaining knives are misunderstood. Honing a knife isn't the same thing as sharpening the knife. The sharp edge is still there it has just simply started to roll away from the true cutting edge of the blade. For example, you can hone a knife but that only results in straightening the sharp blade edge called the bevel.
On the other hand, sharpening a knife restores the beveled edge back to the correct angle specifications for the cutting utensil. So it is possible to sharpen a knife yet provide some honing characteristics in the process of sharpening as knife sharpening also helps to straighten the edge. A good kitchen knife sharpener provides the ability to sharpen the knife, hone the knife, or both.
A two-stage sharpener is very convenient because having the ability to hone a knife blade means that the knife will not require sharpening as often. What determines the angle is what you will use the knife for. The larger the bevels angle the more durable it will be and the longer the knife blade will last between sharpening.
The shallower the bevels angle the sharper and more delicate it will be but the knife blade will require sharpening more often. There are many suitable angles for a beveled knife edge to have but not all are suitable for basic kitchen cutlery. When selecting a knife sharpener it is important to choose one that will restore the kitchen knife to the correct angle.
It is also important to have a knife sharpener that is convenient to use so you feel like sharpening your knives often so that you spend less time on cutting tasks and your knives cut better.