(just a guess) So you can "loosen" the die up to chase existing threads rather than cut new ones. I've found it's sometimes better to have a chaser die than a cutting die when you only need to clean threads.
Everything above is true but the another reason is that when bolts are made at the factory the die used can be either larger or smaller than other dies (within tolerance) thus you can have a bolt that might need to be clearance a little more for it to fit. And there are different fits from interference to a loose fit. So you can change the size if you make a bolt and the nut doesn't fit on you can bring it down some to fit.
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