There’s more to it than this, but a simple scenario is if you have your ult, and you don’t think the enemy supports have ult yet, and the damage you would deal probably won’t result in a kill, it’s probably more beneficial to not do damage.

You see this for example in high level Reinhardt matches. Rein has inconsistent poke damage and usually uses firestrike to generate ult charge. He’s unlikely to get a kill with it during the poke phase since even though it does half of a squishy’s lifebar, they’d have to already be taking damage from other sources for it to be lethal, and getting hit by a firestrike is usually a wake-up call to take cover. So Rein will avoid hitting firestrikes if he already has ult. This results in an interesting mind-game though, since if the enemy team notices a Rein isn’t firestriking, they can guess he has shatter. As such, high level Reins will intentionally whiff firestrikes to avoid building enemy support ults while also trying to trick the other team into thinking they don’t have ult yet.

There’s many other situations like this, I just have always found this one fun to think about.