Advantages of having a snapshot Driver


I have been doing some research on different products performing
snapshotting of volumes. I tried the following out:

  1. Symantec BESR
  2. ACronis TrueImage
  3. And some other lesser known products

I find that most of them implement their own snapshotting driver.

What is the specific advantage of having a snapshotting driver when VSS
provides the framework? I would understand if it was Windows 2000 era, but
none of these products currently even support that OS anymore.

So there must be some advantage over VSS. CAn someone point out them to me