Starcraft 2: Why use Roach over Hydra?

Solution 1:

While it is true that the range 5 (or 6 after upgrading) is better than the roaches' range of 4, and that the DPS is considerably better (~14.5 vs 8 DPS), the roaches still have a number of advantages over hydras.

• Better Health: 145 vs 80
• Better Base Armor: 1 vs 0
• Better Speed post-upgrade: 3.0 vs 2.25
• Lower Cost: 75M 25G vs 100M 50G
• Faster Build Time: 27 seconds vs 33 seconds
• After upgrades, can move while burrowed.

Solution 2:

Probably because Roaches:

• are cheaper;
• don't require a Lair, so you get access to them earlier;
• can move while burrowed (requires Burrow);
• heal themselves while burrowed (if upgraded);
• run faster while not on creep (if upgraded).