Two years ago, smoke was unseen in competitive play. I guess its most obvious use in blocking enemy firing lines was a bit too niche and micro-intensive. Eventually people found a way to protect their superheavies with it, and mortars became a nearly mandatory unit in every opener.
The technique is simple – you smoke on your heavy tanks, and keep them near the smoke. If you see an AT plane, you hide in the puff. Optionally, you can also play peek-a-boo, hiding while reloading between every shot.
Normally, it is suicide to move your superheavy in the open, as it’s guaranteed to be attacked by an AT plane. Using smoke as cover, heavy tanks are no longer restricted to peeking from deep forests. Even if you are operating from a forest, it still makes sense to use this tactic – the smoke is superior cover because it unconditionally and instantly breaks line of sight, whereas forests have a ~350m perimeter where you’re stealthy, but slower to move and still perfectly spottable.
It takes some practice remembering to refresh the smoke, and keeping your heavy close enough so it can retreat to it. That said, in my experience this is by far the most common and useful application of smoke.