A good trainer sock has a bit of structure to it before it's even on your foot, a heel that's shaped rather than just a tube folded over, and a cuff with enough grip that it doesn't roll down into a tight little band under your arch by lunchtime. Cheap ones feel flat in the packet, like a flannel. You can tell before you've even pulled them on which ones are going to last the day.
The thing they clash with is anything they weren't built for, mainly ankle boots and anything with a low vamp, because that's when you get the sock peeking out above the shoe line looking like a slip, which defeats the point of the whole exercise. They're not made to be seen, so the moment they are, something's wrong, either the sock's too tall or the trainer's cut too low, and no amount of tugging at the ankle on the bus fixes it. Colour wise, I've stopped buying anything white after one too many washes where they came out looking like they'd been dragged behind a car, grey and off cream hide the greying far better, and a proper black holds its colour for longer than you'd think, though it does eventually go a sort of dusty charcoal that only you will notice. Nude or skin tone ones are brilliant under trainers with mesh panels where white would show through and look like a bandage. As for when to reach for them, they're a spring and summer thing really, trainers without socks is a look I've never managed without my heels going raw, so these are the compromise, worn for school runs, gym trips, long days where you're on your feet moving between places and need something that isn't going to bunch. They're less at home in deep winter under thicker boots, where a crew sock does the actual job of keeping you warm and these just don't have the coverage.
I bought a multipack years ago that promised to be seamless and wasn't, there was a ridge across the toe that I could feel through to about three in the afternoon every single day until I gave up and threw the lot in the bin. The better ones cost a bit more per pair but the seam sits flat, the heel stays where your heel actually is, and they don't slide down inside the shoe the way the cheap ones do within about four hundred steps. If you find a pair like that, in a colour that matches more than one pair of trainers, you hang onto them and buy the same ones again rather than experimenting.