The Rover is a fast scout vehicle that's cheap to build, and perfect for gathering intel on your enemy. Vehicles tend to have low sight range, and so rely on scouts to spot targets and stop them from running into ambushes.

Use a few of them to take out your opponent's metal extractors at the start of a game.

It's useless for fighting non-scout combat units. However, in large numbers they are an effective way to draw enemy fire or take out unprotected expansions and constructors. Just having them on your enemy's side of the field distracts them from focusing on yours, which, given vehicles' lesser numbers compared to bots, can help you get safely established.