Bombers are an airborne unit in Advance Wars and its sequels. Bombers are very powerful even though they can only attack up close. Bombers can attack Ground and Naval Units and are very proficient at that.
Whenever you create a Bomber, the AI will usually either create an Anti-Air or a Fighter. This is a good reason not to put all your hopes into a Bomber, because it can be shot down easily. However, if you can time it correctly, you can take advantage of that fact and force the opponent to spend valuable funds on a Fighter which you can respond to with an Anti-Air.
Alternately, in a battle that also involves Naval units, the AI could also build a Cruiser, which is also capable of causing considerable damage to a Bomber if it gets to attack first.
If your opponent puts too much into building Anti-Air and Missile Units, Ground forces can be a good counter-balance to the Anti-Air units, specifically tanks, as Anti-Air units don't perform well against Tank units.