I have had my Granite Surface Plate for 6 years. It allows me to make anything flat without the risk of using a glass plate and breaking that. I use it to sharpen and restore older tools for woodworking. With it and good sandpaper you can fix a lot of things.
I use the flatness for two things.
This first is that to properly sharpen a tool blade a straight edge is needed to ensure a clean cut. I can place a blade down and pull it along the sandpaper to make a sharp edge. For a full sharpening start with 400, 600, 1000, and 1500 grit sandpapers from the automotive department (black stuff) at this point the edge is already almost mirror polished and will give a really good cut. I have sharpened almost everything from flat to curved surfaces including kitchen knives.
The second is that to make boards really flat you need flat tools. I use hand planes all of the time and they need to be made flat on the sole to make good joints. These good tight joints make furniture, repairs, and other projects look good and last a long time. It allows me to buy older planes and restore them to service at a great cost savings.
I should also mention that I’ve used it to touch up bad plastic castings. I don’t think any one type is superior as long as it is super flat and able to have a standard sheet of sandpaper put on it.