They protect the tubes from bug impacts (chitin shell remains will stick into the tubes, then proceed to make small cuts in the fork seals, eventually causing them to leak). The seals will be fine for around 10,000 miles before they start leaking, if new. The fork gators will allow fork seals to last around 20,000 miles. Later bikes ('79 onward, for CB750's) did not come with gators, and the fork seals wore out in half the time.
If you leave them off, be sure to wipe down the tubes with a microfiber cloth, after each ride, to help remove microscopic chitin bits from the tubes. This will help the seals to last longer. I'd recommend installing a new pair when the seals are replaced, though. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII