Ever seen people in combat and they run out of bullets at the last minute? The enemy closes in on close combat, the hero it out of bullets. Just as the enemy is about to appear around the corner the hero reaches down and grabs his boot knife in the last second and strikes the enemy in the neck as he does appear around the corner. Victory.
The boot knife can be used as a last resort or a convenient tool in any dire situation. Hunting is a good example. There are various styles of boot knives and Lucid Persona carries them all. Check them out, highly recommended.