Compared to a normal Green Bloon, the Swarm Green card has 20 less health per bloon but has one extra charge. Usually, Swarm Greens are only purposed for aggro decks, especially swarm aggro decks, with the credibility of not being one-shot by double Spike Storm! and having 1 delay.
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.
Unlike normal Greens, Swarm Greens can be one-shot by Thunder Druid's On Play effect, and Firestorm outside of Dark Castle.