Bard Ult

You use it for counterplay.

You can do that on enemy turrets to dive enemy champs nearby.

You can also use it on your own turret if you are defending by yourself & your team is on the way in hopes of saving the turret.

Because It Zhonya's EVERYTHING!

You can stop their turret from firing long enough to dive, you can protect your own turret long enough for your jungler to come in, you can trap enemies under the tower, and prepare followup cc, you can zhonya yourself to make their turret lose focus on you, there is almost nothing it can't do. You just need to time it correctly.

Technically speaking:

for ally turrets

&

for enemy turrets