Well, the obvious answer is, “To sell more doughnuts.”
As to why there is even one National Doughnut Day (not to mention a National Jelly-Filled Doughnut Day and a National Cream-Filled Doughnut Day) … well, this article helps explain the World War I-related origins of at least the first one.
Do you want to know more? Why Are There Two National Doughnut Days? | Mental Floss
There’s only TWO! Now, that’s a travesty. There should at least be a dozen. Get it, a dozen. Come on Dave, that had to bring a little smile to your lips. I’m trying.