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I use it to fine tune my bed height.
You have to use the print settings listed in the description.

Print it out and watch the first layer go down.

As it prints the first layer, you can tell if any of the corners are at the wrong height.
If the corner is too high, the extruded filament looks too flat, and adjacent passes squish together. (Turn that corner screw clockwise a bit to lower the bed.)
If the corner is too low, the extruded filament looks too round, and adjacent passes don't properly fuse together. (Turn that corner screw counter-clockwise a bit to raise the bed.)
If the bed is at the right height, the first layer should be firmly adhered to the bed, and the adjacent passes should be fused together, with just the slightest bit of rounding on top. (Here's a good example I found by Googling '3d printer proper bed height': https://i.stack.imgur.com/AbVDV.png )

I usually tweak the screws while the print is going, and just keep re-printing the first layer until I'm happy with the settings.

Finally, if you let it finish printing, you can use calipers to verify that it's printing correctly on all axes. The different layers are calibrated at .5mm, 1mm, & 5mm, the squares are 10mm x 10mm & the outer square is 100mm x 100mm. The wall thickness is .8mm.