For years I thought black shorts were a cop out, the thing you put on when nothing else in the drawer felt right. Denim ones that never quite sat flat, jersey ones that crept up the back of my thighs before I'd even left the house. It took a decent tailored pair, bought almost by accident, to show me the problem was never the colour. It was that I'd been buying the cheapest version of the idea rather than the thing itself.
A good pair earns its place from about April onwards, on days too warm for trousers but with a bit too much wind for a dress. They're useful for the in between bits of a day, dropping the kids off then straight into something that needs a slightly sharper line, or a lunch that turns into drinks and you don't want to go home and change. Tailored ones do the smart casual thing without trying too hard, and the softer cotton ones are good for travel, because black hides the creases from a bag and the inevitable coffee spill on the train. Where they stop working is anything that needs real formality, a wedding, an evening that wants a dress, or weather that's properly cold, because no amount of good tailoring makes bare legs sensible in November.
The honest limit is also about proportion. Above a certain length they can age you or shorten your legs depending on your shape, and I've learned the hard way that what looks fine standing in a shop mirror can look very different sitting on a low sofa. Worth trying the sitting test before you buy, not just the standing one.
What separates the ones that last from the ones that end up at the back of a drawer is mostly in the things you can't see on a hanger. A fabric with a bit of weight to it holds its shape through a hot wash instead of going limp and see through at the pocket seams. A proper lining means they don't cling in summer. Check the waistband construction, a flat seam sits better under a tucked top than a bulky elasticated one, and metal fastenings last longer than the cheap plastic kind that crack after a few months of being chucked in a bag.
The best ones soften slightly with wear, the fabric relaxing just enough to feel like they were made for you rather than bought off a rail, while still holding their line instead of baggy capitulation by August.