Originally posted by LaurenOE Since the light itself is traveling at the speed of light, then the only important thing, is for the flash to fire at the right time of the shutter.
1/60 or 1/250 or whatever shouldn't make any real difference when dealing with moving objects as the flash itself is fast enough for anything. Am I missing something?
This is true. A technical challenge though is getting equal distribution of that short-duration flash illumination across the entire image -- traditionally this means the flash burst should be timed such that it occurs during a moment when the shutter is fully open (entire film/sensor area is exposed). Mechanical shutters will have a portion of time at the beginning and end of the exposure where the shutter mechanism is still in motion and part of the image is obscured. With a focal plane shutter this is typically a leading & trailing curtain -- one curtain opens in one direction, then a second curtain closes in the same direction. For slower shutter speeds, there is a period of time between the first and second curtain where the entire imaging area is exposed. The maximum shutter speed where this happens is known as X-sync. For speeds higher than this, the trailing curtain begins obscuring one side of the image before the first curtain has fully exposed the opposite side -- so the exposure is a 'slit' that moves across the image plane. HSS is a trick for shutter speeds faster than X-sync where the flash is strobed in multiple lower-power bursts several times during the shutter curtain travel so that the entire frame gets even flash illumination.
If your question is why do we care about 1/60 vs. 1/250 -- the problem is when using for fill flash (daylight sync) if you're limited to 1/60 (or even 1/250) you may be forced to stop the aperture down a bunch to avoid overexposure -- this makes it difficult to shoot with large aperture (for depth-of-field management) + flash in daylight. You can use a neutral density filter to cut the light but that cuts your available flash power dramatically as well. On the Q in particular, you want to shoot with larger apertures most of the time to limit the effects of diffraction and preserve the already very limited ability to blur backgrounds.
Also, most exposures are a combination of flash AND ambient light. The lower the shutter speed (e.g. 1/60 as you mentioned), the greater contribution from ambient light. This may mean that any subject motion may still produce some blur -- the main exposure from flash might look pretty sharp but there may be some blur trails to/from that main exposure.