Gauge wheels allow you to float the implement so you don't have to worry about your 3PH height. The wheel bushings will dictate the finish blade height. For instance, if you set the wheel bushings for 1" blade clearance, then the blade will carve into anything that's more than 1" off the ground. Gauge wheels will allow the implement to follow the contour of the surface more closely. Without gauge wheels, your implement height will be dependent upon the contour of the surface your tractor is riding over. For instance, if you start to go over a bump with the front of your tractor, it will send the blade into the ground without gauge wheels. With gauge wheels, the blade would continue to maintain the proper blade height.
It's the same reason why so many people recommend gauge wheels on landscape rakes. Of course, there may be instances where you don't want gauge wheels on a blade. For instance, if you are trying to shave a few inches off of a grade, you need to be able dig in; in this case, you would need to lose the gauge wheels. This is very simple to do, you simply pull the pins and remove the gauge wheels and bushings.
With all of that said, I can't say that I have seen to many blades with gauge wheels. I have seen a box blade with gauge wheels on this site. I guess the need for gauge wheels would depend upon what you plan to use the blade for. What do you plan to do with it?