Bob Atkins has written a tutorial explaining how a DSLR camera’s shutter speed curtain works and how this limits the highest flash sync speed possible to about 1/200 sec.
It can get a bit technical but what you should retain from it is that if you want to be able to use flash in bright daylight, your camera needs to support high-speed sync flash. Why would you ever want to use high-speed sync flash in daylight? To fill-in your subject using flash. But if you also want to use a large aperture to blur the background, you now have a problem because a large aperture means that you need to use a fast shutter speed, usually at around 1/2000 sec., much faster than the 1/120 sec. a normal sync is. Hence the need for high-speed sync.
If you want the whole technical treatise on the subject, be sure to read the tutorial at: Bob Atkins.