A perk associates a player with some capacity enables during a few seconds to try to make a strategic difference against its opponent.
The perks are thought according to the gameplay so as to make it more tactical. Activating a perk doesn’t necessarily aim to make frags.
For example in the Capture The Flag mode : teams are going to be organized with players in attack to capture the flag, others in support of the attackers or certain locations, and others in defense.
The perks are designed to serve those functions. So for example, a flag runner could teleport himself, a support could heal, and a defender could protect/block a zone.
Here is the philosophy of Master Arena. An old school shooter with modern tactics.