Deflection Shields are used generally with Disruptor Turrets. They do not prevent units from doing damage to the owner's ships and other base defenses, they act as a 'recharger' unit. The enemy must bring more fleet to the battle to take the base in a single attack wave. For example, think of a scenario where just the right amount of cruisers one hit killed Disruptor Turrets. Now, if there were a set of Deflection Shields there, the damage done by the cruisers will spread between the shields and turrets. After the combat round, the HP of the shields and turrets are also redistributed and might "revive" some of the destroyed turrets. These, like Disruptor Turrets, are usually replaced for their better version, Planetary Shields.