Common?
Optics.
Combustion engine cases. They have to contain very high pressures. The connecting pieces have to be milled flat for the gaskets to be able to contain the pressure.
Firearms are not actually up there. Well, not common firearms. Many firearms do require very low tolerance in certain parts, but you could call that more of a design shortcoming than a necessity. Some of the more modern versions of handguns and rifles are made more production friendly by smarter design.
Anything that spins at high rpm, yeah. But if other than balancing, if your device requires super precise custom parts, anymore, it might be more of a design bug than a feature. Even jet engines turbines spin around on?? factory made ball and roller bearings that aren't much different than what you can buy on Ebay or from McMaster Carr. Fundamentally the same things that are in your $20.00 skateboard. Ball bearings, roller, bearings, one way bearings. Dozens of different kinds of bearings. Bearings are one of the most common high precision things we commonly use, and they've been standardized and mass produces for decades, now. The amount of precision and low tolerances in a 3 dollar ball bearing put to shame anything you will produce straight off a milling machine.
Increasingly, timing things with specifically shaped cams or gearing can in many cases be replaced by electronics. And the storage density of li ion batteries are making small combustion engines more and more a curiosity from a past age.
If you are looking for nails to hit with your hammer, the most killer app for a metal lathe, specifically, is threading custom parts and making gears, IMO. Of course, the most basic functionality will be boring and shaping. For a mill, the most fundamental basic utility, IMO, is facing and slotting.