From Destinypedia, the Destiny wiki
The raw, brute strength of a Titan's melee offers a full-force response to immediate danger —- perfect for quick strikes when overrun or when taking the fight directly to the enemy.
With their finely tuned reflexes, Hunters are naturally gifted with knives. In the right situation, a sharpened blade can make quick work of any enemy.
A Warlock's curiosity can often lead to interesting predicaments. To aid in their survival beyond, the Orders learned to harness the physical world into quick, focused bursts with deadly intent.