Barrel

Overview
Barrels (explosive barrels) are red oil drums found in virtually every level. They have a low amount of health, being easily destroyed by any weapon, and upon destruction will explode, dealing heavy splash damage in a large radius (about 4 tiles).

Baiting
Barrels can act like Mines, sitting there waiting to blow up on the unsuspecting tank. You can utilise this by alerting an enemy, and shooting the barrel when they come close. Make sure you're at a safe distance (about 4 tiles away) to avoid damage to your own tank.

Destroying walls
Barrels provide a lot of tile damage too. If a barrel is nearby a tile you wish to break, shoot it instead; it will be quicker to destroy the barrel and any nearby tiles will blow up with it. If you have a powerful weapon and the barrel isn't close by, don't bother just shoot your tile.

Conclusion
They may just be a simple tile, but Barrels can be quite handy. If you use them you can get a suprising amount of good out of them. Just don't stay too close when doing so.