Let's cut straight to it - there are only two reasons you would need to use high-speed flash sync:
to have an appropriately shallow depth-of-field,
to have a sufficiently high shutter speed to freeze action.
That's it. Just those two things. When you need shallow DoF, or a faster shutter speed than max sync speed, then you go to high-speed flash sync.
What HSS doesn't do - it doesn't allow you to overpower the sun. When you go to from normal flash mode (at or below max sync speed), into high-speed flash sync, then you lose Read more inside...