Leggings are the thing you put on when you need to move, sit, stretch or lounge without your trousers fighting you. A proper pair works as hard under a long jumper on a school run as they do on a run itself, and the difference between a good pair and a bad one is never obvious in the shop, only an hour into wearing them.
Where leggings fall apart as an outfit is when they're paired with anything that ends at the exact same length as them, a cropped jumper that stops right at the hip can leave you looking like you've forgotten the bottom half of your outfit, and a top that's too fitted on top of fitted bottoms just reads as loungewear rather than an actual outfit. Thin, see through fabric is the other big clash, you find out it's see through in daylight, not in the changing room, and by then you've already worn them to work. On colours, black is the easy one and there's a reason it dominates, it hides bobbling and goes with nearly everything, but a good bottle green or a soft grey marl is worth having too, they read as proper trousers rather than gym kit once you're out of the house. Avoid anything with a shiny or coated finish unless you specifically want that look, it catches light in a way that's unforgiving on anyone, and it was never designed to be flattering, just visible on a dark running route.
For wear, leggings earn their keep most in fitted jumper, cold weather, errands and school run season, and separately in anything that actually involves moving, gym sessions, a run, a long walk where you don't want fabric twisting round your legs. They're less convincing dressed up for an evening unless the rest of the outfit is doing serious work, a long blazer and proper boots can get you there, trainers and a hoodie won't.
Fit is where cheap ones show themselves fastest. A bad pair goes see through at the thigh within a few wears, the waistband rolls down by lunchtime, and the fabric goes baggy at the knee after one wash. A good pair holds its shape wash after wash and the waistband sits flat without digging in or sliding. Go by your actual measurements rather than your usual size, brands vary wildly, and check the waistband height, high waisted ones are far more forgiving if you're sitting down a lot or bending over. If they're sheer when you pull them tight across the thigh in the shop, they'll be worse once worn.
Reach for leggings on any day that needs comfort and movement without looking like you've given up, school runs, gym days, long car journeys, or simply cold mornings when trousers feel like too much effort.