That is, rather than find the quickest route from A to B, Chekov might concentrate on monitoring nearby traffic, adjusting the navigational deflector to balance protection-of-ship vs. drag-induced-by-deflection, or running the sensors that tell where A and B are in the first place. Or whatever.
Well we do know from "Squire of Gothos" that the navigator does use the sensors to check their flight path for hazards and space density. So the navigator could also be responsible for finding the fastest (or least resistant) path from point A to B. We've seen in "Balance of Terror" that the fastest path isn't a straight line for the Enterprise.
The helmsman could also do that to since they seem to have overlapping "plot a course" orders given to them.
Perhaps it's added combat redundancy or a way to give additional flight hours training to aspiring helmsman or science officers by working their way through the navigator's spot.