Blizzard is a heavy and slow bomber that can deal great amounts of damage flying over units and structures.
It is mostly designed to destroy buildings, but used well it can also deal high damage to mobile units - try to intercept their movements.
It deals much higher damage than the T1 bomber Stormbringer. As little as 4 Blizzards can destroy a Fusion Reactor on one flyby (7-8 for an Advanced Fusion Reactor). They can drop another round of bombs every 7 seconds.
They are cheaper but weaker than the CORTEX counterpart - Hailstorm.