Sparks have large wing size, shields, high alpha strike, burst recharge mechanic, high speed, long attack range with perfect accuracy, very high armor penetration for a fighter, and don't cost crew. Their only two downsides are that they have a good instead of great replacement time, but the Drover ship system nullifies that (and increases wing size), and that they do soft damage to shields - but that only matters when they don't have the numbers to overwhelm the shields.
Their high alpha strike, long attack range, and perfect accuracy make them the perfect fighter to swarm with, while their lack of crew removes any logistical penalty for doing so.
Quick calculation for the alpha: 3 charges stored, 1 recharges in the time to fire 3, so the first 4 shots are at full dps, at 128 damage per shot and 100 shots per minute: 1.8 seconds at max fire rate (first shot at time 0). In that 1.8 seconds a single spark wing unloads 4*5*128 = 2560 soft flux damage per wing. Even frigates can tank that on the shields (if they have good arc), but consider what happens if there are 4 wings (IE a single legion): ~10k damage = frigate instantly dead, destroyers completely fluxed out. Now consider say 10 wings, boosted because Drovers: 4*7*128 ~ 36k damage in 1.8 seconds. When that is concentrated on a single target even capitals are in trouble.
Any time the wings happen to be off target/unable to fire, this alpha burst is recharging. This might sound like it doesn't matter, but watch some broadswords attack a target: they spend a lot of their time not pointed at it and firing, because they are maneuvering. Sparks use that time to recharge their shots.
BurstPD has according to the chart 350 damage per second, halved because its a beam, for effective armor penetration of 175. That better penetration than a Warthog's light mortar, despite that being an HE gun!