The Military Art of Training, published in 1622, recommends that soldiers wear a dagger, describing it as "the necessariest weapon that belongs to a souldier." Six reasons for doing so are listed:
(1) The handsome appearance of the weapon;It's easy to think of additonal reasons that could be added to that list, but it's a good start. Reason (4) reminds us that the type of dagger called a misericorde ("mercy stroke") was intended specifically to finish off mortally wounded combatants. Reason (5) was a popular reason for wearing a bowie knife--plunged into the ground, it provided a handy stake with which to tether a horse on the prairie.
(2) Its advantage over the sword at close quarters;
(3) Its superiority in a private combat;
(4) Its utility for the speedy despatch of the vanquished;
(5) Its advantage as a means for tethering a horse in the open;
(6) Its awe-inspiring effect if drawn by an officer when sedition is imminent.
NOTE: The six reasons quoted above were from a secondary source; I haven't been able to access the original text. However, I later found it quoted elsewhere in language that sounds like an exact quote and more authentic to the period. These, then, are the six reasons that a soldier must wear his dagger:
1st, for ornament; 2ndly, for use in the melee, that when he cannot use his sword, he may doe good with his dagger; 3rdly, if it should come to a private combat, and a sword should break; 4thly, for despatch of the vanquished; 5thly, for tying a horse in an open field, where there is neither bush nor hedge; and 6thly, for the punishment of offenders, for a captain or inferior officer that only draws a dagger, may appease a sedition.