Millimeters to inches is a workshop and manufacturing staple: a huge share of tools, drill bits, screws and lumber sold in the US are sized in inches (often fractional — 1/4 in, 3/8 in), while metric-designed parts, sheet stock and 3D-printed components are specified in millimeters. Getting the conversion slightly wrong is the difference between a bolt that fits and one that doesn’t.
A useful reference point: 10 mm converts to 0.3937 in — just under 4/10 of an inch, which is why a “10 mm” socket and a “3/8 in” socket are close but not identical, a common source of stripped fasteners when someone grabs the near-enough size instead of the exact match. At the smaller end, a 6.35 mm gap is exactly 1/4 in (0.25 in) — a clean, exact match, since 0.25 in is itself defined as exactly 6.35 mm.
This conversion matters most where a mismatch is invisible until it fails: fastener threads, drill bit diameters, and tolerances on manufactured parts crossing between metric and imperial supply chains.