So that the harmonization point may be independent of the horizontal convergence point.
Actually, if you set your convergence at 500 yards, your rounds will cross horizontally near 500 yards and will cross your gunsight center vertically also at 500 yards
With the current system, if you want your guns to fire a bit higher (quite important unless you are firing from the dead six of a nonmoving target) you need to set your convergence as far as possible, lets say 600 yards. But the fact is that you are going to open fire at much closer range, for example at 300 yards or even less. Your rounds will converge horizontally at 600 yards and will be quite separated at 300, where your target is, so you will not be able to concentrate the fire in one spot and your hits will be too disperse to cause critical damage with a single short burst.
What you really want is to set the harmonization point at 600 yards and the convergence point at 300.
Also fix the convergence of paralell guns like the 190s Mg17 and Mg131. These nose mounted MGs didnt have any horizontal convergence point as they fire paralell to each other.