Most 3d printers overshoot their wall widths while printing, making components just slightly too big to really fit each other. For this reason, a lot of engineering parts have some sort of tolerance gap that you can set. However, printing 10 non-fitting parts just to get the right value for the tolerance does not make sense. This is an attempt to get the right value first time. Or to tweak settings for years on end to minimize the ooze. Whatever floats your boat.

This is part of a planned suite of calibration test prints to fine-tune your 3d printer to print accurate 3d components to a level where the components will fit each other without post-processing.