A very high quality binocular stereo microscope, switchable to either 10X or 30X magnification, built in light.
At 10X, better than the best loupe or 30Xfor for exceptionally detailed inspection.
You can use the best jewellers loupe, you can stare at a stamp or coin for a very long time and not be sure if it is a forgery. It is better to use a microscope, best of all a stereo binocular microscope like this (the least tiring on the eyes), more powerful than any loupe but not too powerful (you still need to see the detail of the design).
You've stripped the watch and you can see, through an eyeglass, that a jewel hole is not quite the shape it should be, but no matter how good the eyeglass you won't see the problem - through this you will instantly see if it's worn or chipped, how bad the damage is, and whether it merits replacing.
You've carried out the visual check on the PCB, it looks OK, but you suspect there's something wrong with the soldering process. You use a handheld magnifier but it's useless, you try a watchmakers eyeglass but it's not powerful enough. Only a microscope will show you if the joints are consistent.
You are checking plant or insect samples, you've found two that are so similar that you can't tell them apart, and the best quality loupe isn't clear enough. You need to be able to see the entire sample, the 'biological microscopes in the lab. are just too powerful. This is the answer, a relatively low-power inspection microscope.