Primarily it's before the roll - which basically allows you to control the difficulty of your rolls, but with a limited resource pool to spend to do so. It also reinforces the game's core underlying themes of economic scarcity and risk-vs-reward (so "How much do you want to gamble on this roll?") I know there is an upgrade for some weapons to represent automatic fire, which allows you to spend charges AFTER the hit in order to up damage (normally extra charges spent on rolling to hit only improve your chance to HIT, not improve your damage).