To be honest with you, not much usefullness unless of course you are shooting with flash outdoors very frequently and using wide open apertures. A flash sync of 1/250 sec is sufficient to freeze anything that moves. In fact, it is the flash that freezes the subject and not the shutter speed.
With 1/500 sec sync the advantage is that if you need to do a fill-in flash on your subject outdoor but do not wish to stop down your lens aperture, you are able to do so without much restrictions.
Here's an example, if shooting at ASA 100 on a bright sunny day with shutter speed at 1/125 and aperture of f/16 with a hotshoe mounted flash. The flash may not be powerful enough and produces a lot of hot spots and yet still don't really fill-in the shadow areas. Depth of field is too much also due to the stopped down aperture. But if you have a 1/500 sec sync speed, you can always open up the aperture to f/8 for the additional power and shallower depth of field.
To be honest with you, most hot-shoe mounted flash don't do a good job of providing ample light at even f/8 so that is why corporate and commercial photographers uses studio strobes.
High Speed Sync modes require the flash to fire a high frequency burst and hence have much reduced working distance. X-sync speeds should allow you to use your flash at full power up to its rated speed.(1/500).