Knee high socks exist to fill the gap between where a boot ends and a hemline starts, keeping bare skin out of the cold without you having to commit to tights for the whole day. They're a layering trick as much as a sock, the kind of thing you pull on under a skirt in October and forget about until someone asks why your legs look warmer than theirs. I pair mine with a-line skirts and shorts over the top in that in between season when it's not cold enough for full coverage but definitely too cold for bare ankles, and they work just as well under a dress with ankle boots, the sock peeking out like it was planned rather than a last minute rescue from the bottom drawer.
If you've never owned a pair, start with a plain ribbed cotton blend in a dark neutral, navy or charcoal rather than black, because black knee highs have a habit of looking like they've wandered off from a school uniform. Do the stretch test in the shop, pull the top band between your fingers, if it springs back sharply and feels like it has some elastane running through it, it'll stay up past lunchtime. If it feels floppy and cotton only, it'll be round your ankles by the time you've walked to the bus stop, which I learned the hard way standing on a platform fishing a sock out of my boot like I'd lost a fight with it. The honest limit is that they're not built for all day standing or for anything where the top band will show under trousers, because nothing ruins a smart work look faster than a visible sock line through tailored fabric, and they're a poor choice with anything sheer or fitted where the band leaves a mark on the leg by mid afternoon.
They come into their own with boots and skirts from autumn through to early spring, and for the odd warm day with shorts when you want the look of bare legs but not the actual draught.
Wash them inside out to stop the ribbing going baggy before the elastic does.