You’ll want to use high-speed sync when you’re using a high shutter speed that is faster than your camera’s flash sync speed.
An HSS is also very beneficial for scenes with a very bright background, where your subject is in the shade. Think of outdoor portraits, for example, when you’ll want to use a wider aperture.
A wide aperture to keep a shallow depth of field will force your shutter speed to around 1/800th of a second.
At this fast speed, you will not be able to sync your strobes or flash units, unless you use the high-speed sync technique.
The only way to capture the subject in the shade with a correct exposure is to use high-speed sync. You’ll have more control over the light.
You wouldn’t be able to do this without high-speed sync as the background is brighter than 1/200th of a second.