The nature of flash lights is a burst of light.
To accomplish that the shutter has to be completely clear of the sensor.
That dictates the max sync speed of a camera.
If the combination of ISO, Av and Tv (at or less than sync speed) do not provide a good exposure you have to do something.
HSS permits using a Canon proprietary protocol to strobe the Speedlite to compensate for the shutter curtains moving across the sensor. Since the Speedlite is strobing it can not provide a full power burst.
The other option, made possible with Hyper Sync, is to use the less than full power tail of a monolight that extends long enough for the shutter slit to cover the sensor. The disadvantage is that since the light is diminishing over time, the illumination is uneven.
Your choices are manual below sync speed, manual with HSS, or manual with Hyper Sync. It all takes some experimentation and testing. Just like everything else. 