No part is ever exactly the size it says
No manufacturing process produces a part at the exact dimension written on a drawing every single time. A hole specified as 10mm might actually come out anywhere from 9.98mm to 10.02mm depending on the machine, the material, and the process, and that small window of acceptable variation is called a tolerance. Tolerances aren't a sign of sloppy engineering, they're a deliberate, specified range that reflects what a given manufacturing process can realistically hold, and tighter tolerances generally cost more because they demand more precise equipment and more careful inspection.
For a static part like a housing that just needs to hold its shape, loose tolerances are often fine. For a joint on a robot where one part has to rotate or slide against another, tolerances become a functional requirement, not just a quality metric, because the actual gap between the two mating surfaces determines whether the joint works at all.