It can be a positive Elixir trade for the Poison card if your opponent places it down. It nullifies the Poison's effects for the most part as the healing per second of the Heal is greater than the damage per second of the Poison, but at equivalent levels the Poison will do more total damage than the total healing capability of a single Heal, because the duration of the Heal is much shorter than that of the Poison's.
The Heal Spell is also effective against the Fireball if the Fireball doesn't one-shot the troops.
The Heal Spell can heal low-health troops when they're targeting an arena tower such as Wizard, Musketeer, or Inferno Dragon to continue dealing heavy damage towards the tower, this will surprise your opponent.
Healing troops that the opponent would normally ignore and let the Crown Tower defeat can catch your opponent off guard and result in the Tower taking significant damage.
While the Heal has healing capabilities, it does not heal any troops that have been destroyed. Also, it cannot heal troops over their original health. Once the troop has been fully healed, it will not be affected by the Heal. In fact, if the troop is eliminated by a particular attack in one hit, the Heal will be incapable of doing anything. For example, you cannot keep Skeletons that are under an enemy Poison alive using a Heal unless the Skeletons are sufficiently leveled such that they do not die in one pulse of the Poison.
A possible strategy would be to use a Miner and a Minion Horde with a Heal, but only if the opponent does not have Arrows or any other high-damage spell, excluding Lightning. This would work since Minion Hordes are deadly, so the opponent must defend it. If they use a Zap, a Princess, or Ice Spirit, the Heal will keep the horde alive and shredding the tower. This would work with a Goblin Barrel too if the opponent's counter does not one-shot the Goblins.
Heal was featured in its own Special Event Challenge, the Heal Draft Challenge, which started on 28/4/17. By achieving 6 wins, a one-time reward provided the player with Heal cards, with more Heal cards obtainable at 12 wins as another one-time reward.