A thickness tester makes plywood joinery easy, accurate and predictable.
One of the great questions every woodworker asks is why wood you buy from a home center is never its stated thickness. You already know that 2 x 4’s are actually 1-1/2” x 3-1/2”. At least you can depend on lengths being accurate. Far worse is plywood. 3/4” plywood is only in the range of 3/4”. Manufacturing tolerances are supposed to be: plus/minus 0.8 mm (1/32 inch) for some soft plywoods and plus/minus 0.4 mm (1/64 inch) for utility grade plywoods, to an entirely different set of tolerances for hardwood plywoods: plus 0.0 mm/minus 1.2 mm (3/64 inch).
Besides thickness standards being confusing, there are added variables of temperature, moisture content and significant manufacturing variations from batch to batch.
The result of all this is that a woodworker can’t depend on any stated plywood thickness. If plywood thickness is a critical dimension to your project, you always have to measure it and test it.