Fireteams of 3 or 5 members take a single action with the leader getting assisted by the other squad members. This is in effect a net loss of two or four actions. In return you have a guaranteed hit (most likely) and a couple of momentum spends for every attack action.
Mechanically it’s less overhead for the dm, and a fireteam is a nice visualization for the players. Without any increased damage or attack per fireteam member, it’s hard to feel that this setup is very optimal however . The players would start shredding fireteams, is that intentional?
Dealing with each member of a team would get quickly bogged down an make the game a bit too slow for many where the possibility of fire teams spending effects for multiple hits on people in the area deals with this beautifully
The higher likelihood of hitting, and the extra Momentum generated from those attacks is the increased damage (or, rather, increased effectiveness). Functionally, it’s the same as one NPC being assisted by up to 4 other NPCs. As those NPCs would be largely ineffective anyway (only rolling 1d20 on actions), you’re trading several ineffectual actions (attacks that would’ve missed, etc.) for one action that is likely to have an effect.
And the PCs are meant to chew through fireteams of trooper NPCs. They’re the nameless mooks who basically only exist to get shot in the face by the protagonists. They’re often more akin to ambulatory terrain than meaningful opponents. It’s the Elite and Nemesis NPCs who are meant to be a threat.
Fireteams are also a great way to get past your PC’s defenses, which I’ve found can be substantial in this game.
For example, a PC wearing an ODD device or using any type of Thermo-Optical Camouflage is essentially immune from being hit by a Trooper. Throwing them up against fireteams allows you to use Troopers in your combats, but still present a challenge to your PCs.