Its helpful for centering "new" base plates for screw holes, and when using template guides. some bit diameter to inside guide diameters can be very tight, an offset of 1/128" can mean disaster.
I can't speak for all 2nd party plate manufacturers but all the extra non (OEM) plates I ever bought don't come with alignment ribs like the PC default plates do. As tight as the (default) plates fit even they need to be checked when tight tolerances are in effect.
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