If you're making a sieve and the only function it has is to sieve, it doesn't matter if the holes are a bit too big or a bit too small, as long as it sieves. You can pick a supplier on price all day.
But if you’re manufacturing complex components for a Space Shuttle you need precision parts engineering from an experienced company you trust. It is the same for any complex component in any industry. You need the part to work as designed. When lives depend on a component standing up under stress it must be manufactured to spec and precisely as per the drawing!