That's not a dumb question at all. Most lanyards are merely ornamental, being too short to be useful.
If you have a tip up carry folder with a pocket clip, the lanyard can be used to help draw the knife from your pocket. If you have a fixed blade knife, the lanyard can be used to draw the knife from the sheath. A lanyard can also be used to secure a knife to a loop on a vest or jacket, to keep it from getting lost. You can also run your wrist through a looped lanyard so it doesnt get dropped into the water during use.