The Valiant creates a protective bubble that envelops your nearby units. Damage taken by units within this area-of-effect is redirected to the Valiant.
Valiants are key to protecting valuable units. Try to build several of them as early as possible.
One Valiant can take damage for a large number of allies at once, which makes it an efficient way to absorb AoE attacks.
Keep in mind that once a Valiant stops moving, its protective shield takes a few seconds to form.