That one Jack's got might actually have a use.
The one on this
was designed for aircrewmen to saw themselves out of an aluminum airplane, after it crashed.
are just to look bad-ass. Theoretically you could cut a branch with it, if you were trying to survive in the woods, but usually those teeth don't work very good.
Back in the olden days, this knife would be useful to a swordfighter. He'd have a sword in one hand and the knife in the other, and he could use the knife to catch the other guy's blade. If he knew what he was doing, he could break the other guy's blade.
Meddle not in the affairs of dragons, for thou art crunchy, and taste good with catsup - George of Lod, Year of Our Lord 297
I always take precautions.
Beware the Evil Bullet Fairies.