It served obviously as a tactic which bolstered defence by mobility, multi-directional archery, and could have been "economically positive" in adjusting firing rates (Support archery from the back release in organized volleys, while these circling light horse fire from all possible frontal angles); with such a military philosophy in mind, the mobile nature of this doctrine would also allow for riskier tactical movements, such as treading forth for an even clearer shot... Or as a lure, in order to retreat with a parting shot (The "Parthian Shot" as given by the Classics). Foremostly, the formation creates a dust which is difficult to screen to the opposing party. We can allude several lessons from the battle of Carrhae, with sun gleaming on the cataphract armour and the dust which the horse-archers kicked up in their feigned retreat. Immensely important tactical tidbits.

The downside, as we must take into account, for any mounted unit organized and fitted without stirrups or at least the Sassanian-type saddle, are two-fold: Fatigue, no matter how hardy the Iranian light horse were, was always a liability. Second of all, perhaps even more dangerous, heat-loss, and cold-legs. Eventually, it could get so bad that the knees which nomads were famous to use in order to steer their mounts, without having to use their reins or a whip, could be incapacitated. Now of course, light cavalry tend to be quite agile and move around a lot, riding side- or even back-saddle... But in the tactical end of the matter, cavalry of any type is always more time-sensitive than infantry. This is largely in due to the eventual unpredictability of the horse itself, and its conditioning.

The origins... Well, obviously, we can be quite confident in that the tactic is about as old as the adoption of the horse in steppe warfare, and over time refined into a tactic suitable for light horse armed with bows and/or javelins. After all, what use is a hundred horse-archers, costing several times more than a hundred foot-archers if their tactical span are identical? Foot-archers fire in volleys or platoons because they are limited in their role; horse-archers firing entirely in organized volleys seems rather incongruent with their mobile nature, and does not fit historical reality.