FOR SHARPENING HOOKS (IN THE EVENT YOU DON'T WANT TO MISS FISH)
Fact: you're more likely to cut yourself with a dull knife. Another Fact: you're more likely to miss fish with a dull hook. Think about it: your fly has a ton of opportunities to become dulled even if the hook was sharp when it left the manufacturer. Why not put your fly in the best condition to catch fish by sharpening it?