Personally it means I can adjust flow rate, and shutoff pressure quite easily. Helps when you are doing leaky georgians, put shut off pressure to 7, and flow to 1, and you can be very careful.
Even with a flow valve, you cannot always get the exact same flow rate everytime you adjust it? Never used anything but varistreams I must say though, so I might be wrong.
Also you can quite easily tweak your system to just the way you like it, you can with a shureflo as well, but it might not be as easy as pressing a couple of buttons.
Apart from that, not much I don't think. When I got one, there was still a debate over how long a pressure switch would last as well, which led me to purchase one.