Cats very often do not drink enough, as they lack the strong thirst mechanism that dogs have.
Neutered male cats have quite a small urethral opening. Not drinking enough AND eating dry food only combines to make urine quite concentrated, and crystals can form and cause blockages if they cannot pass.
Canned food contains a large amount of water, which aids in keeping urine dilute.
Canned food is NOT any worse for teeth than dry. This is a myth perpetuated by makers of dry food. I prefer my cats to eat mainly canned food.