Grunt is a light infantry bot which is cheap and quick to build. Proves useful for scouting and taking down unguarded structures like metal extractors and early eco. It is armed with two light, but precise lasers with a slightly greater range than Pawn, so with little micro-ing skills you can outrange them easily.
In late T1 warfare Grunts can be used for ambushing Commanders and speedy skirmishing. However, light armor and relatively short range makes them very vulnerable to defensive towers like Guards and Riot tanks.