10 microns = .01 mm = .0004 inch
I used to work for a company that made stamping dies for aluminum cans, and some of those dies had tolerances close to .0004", because the aluminum is very thin and could crack and tear if the dies were not made precisely. The cans themselves are not that precise, they just need to hold beer without exploding. I can’t speak to Legos, but cars absolutely do not need this kind of precision, not even in the bearings. And especially not in the sheet metal body panels.
I can’t speak to Legos
Legos famously have a weirdly high tolerance for injected molded plastic*, it’s part of the branding they use to justify their high price. It does make them snap together more reliably than Mega Blocks or whatever, but Mega Blocks or whatever usually snap together anyway, so I don’t know whether that extra precision counts as necessary.
* People quote all sorts of tolerances for this, but the most credible-looking one I found was 0.04 mm.
Tolerances depend on the function of the part and are selected to balance various tradeoffs in production costs and assembly. A well engineered design does not require tight tolerances for the vast majority of features (reducing scrap, tooling, and labor costs), but some specific mechanism components like gears and driveshafts demand very tight tolerances for profile and runout in order to function reliably.
Tolerances will often influence which type of machine tool is used to produce a feature. A tight tolerances on an outside diameter might make the difference between a part being made on a lathe in one/two operations, or requiring additional operations on a cyllendrical grinder. Overzealous requirements for surface finish will require slower feedrates, sharper tools (which wear more quickly) and extend cycle times significantly, or require extensive manual hand-finishing.
Commercial bearings routinely have tolerances of 0.0004" or less & performance bearings designed for specific aerospace use/applications can have substantially tighter tolerances.
