Post-match analysis.

I often notice that when playing as the Swamp faction, players build and upgrade Lizards to Lizard Warriors as a priority. Why and when is this done, and in what situations?

On the first day, they build flies if the heroes don't have them. Otherwise, upgrading is essential if there are at least 20 Lizards...
As mentioned, there are many advantages: damage, speed, 24 shots, and hero specialization!!
And there's another controversial but quite real trick!!!
I've played over 100 games as the Swamp faction, playing as Vistan, and noticed that upgraded archers more often deal damage close to the maximum compared to un-upgraded ones. Monst confirmed this, playing as Jabrakras with Barbarians!!!
Upgraded archers have more consistent damage!!!