Deflector Shields are often used in sniper fleets because most weapons that can fire back at destroyers are Projectile-type.
They take up the least mass out of the three basic shields; this allows for a balance between shielding, armor and damage output.
Deflector Shields have the least shield energy, making them relatively vulnerable in fleet combat since hardly anyone uses Projectile weapons. The ones who do most likely have Phased Projectiles, Phased Magnets or Reinforced Rails equipped as well, severely reducing the effectiveness of your shields as most of the damage ignores shield protection.
Hulls that have active Deflector Shields glow light-blue.