“Machined” doesn’t mean “homemade”, it’s a manufacturing method meaning that the threads of the bolt/nut were cut from a smooth piece of metal (as opposed to being cast or forged from a mold). Machined hardware is more likely to have defects like tiny nicks in the threads or bent threads that can damage/ destroy whatever you’re using it to fasten. You can usually tell pretty quickly if you’ve got a shitty bolt or nut if you’re threading it by hand - any irregularities are easy to feel. But if you just blast it with a power drill right off the rip, you won’t feel any of it, and might end up with an unplanned permanent fixture.