Corvettes are a light attack ship which can move between planets. This allows you to use it for scouting when Scout Ships are unavailable. They are also useful as fast strike ships, which can be very effective at hitting Recyclers and attacking bases without defences. Paired with Destroyers they can form a strong force against Cruisers and lower.
In the early stages of a game players may send lone Outpost Ships to colonize bases. These are super vulnerable to corvettes, unfortunately they lose that use as in the later game, as people usually protect their Outpost Ships. However, corvettes have laser weaponry like Fighters do, which is usually researched to high levels; thus they could be considered mobile anti-aircraft platforms.
However, because a Corvette only has exactly twice the stats of a single Fighter, it would only take two Fighters to destroy a lone Corvette. All things being equal, there might be mutually assured destruction (both the Fighters and the Corvette will be destroyed), and the attacker with the Fighters will come out ahead in terms of credits lost; 10 for the Fighters as opposed to the 20 for the Corvette. This is assuming that the attacker and defender both have equal laser and armor technology.
How to counter: Capital ships.