Compared to a normal Green Bloon, the Swarm Green card has 30 less health per bloon but has one extra charge. Usually, a deck for Swarm Greens should pair it with multiple normal Greens.
This is the only Bloon card with both three charges and 2 Gold cost per bloon. In the earliest rounds, this is more reachable than, for instance, a Yellow then a Yellow next turn.
For instance, one Swarm Green can be played first and followed up next round with two Swarm Greens.