I'm just guessing, but it would probably be more portable to different platforms – the only thing each OS would need is its own local server and (probably full-screen) web browser.

Also, it might be nice for a manager to be able to look up how sales are doing when they are not in the office. Of course, this could be done without making it totally web-based, but it would be harder.

It might be easier for users to customize the system, too – it is easier to change the GUI of a web app, I think (like adding more buttons to a window). The users could also perhaps use a custom stylesheet in the browser if they didn't like the look, though ideally the PoS app would allow changing the look from from within itself.