Leggings are the thing you put on when you want to move properly, whether that's a run, a yoga class, or just the school run without your jeans digging in on the car journey. They've taken over from jogging bottoms for a lot of women because they're warmer against the skin without the bulk, and because a decent pair holds you in rather than just covering you up. That's the whole job. Comfort with a bit of structure behind it.
Where they go wrong is in what you put them with. A thin, slightly see through pair under a short top is how you end up finding out the hard way in a work meeting or at the school gates, so always do the bend over test before you leave the house, not after. Chunky jumpers with a baggy fit don't do you any favours either, you end up looking like you've forgotten your trousers. Nice with a longer line top that covers the bum, or a proper coat that gives the whole thing a shape. Colour wise, black is the one that forgives everything, which is why I own about six pairs and can still never find a matching one on a Tuesday morning. Navy and a deep bottle green are nearly as kind. Anything pale or bright shows every seam of your underwear and every change in the light, so save those for layouts you've actually tried under your own bathroom bulb first.
I bought a cheap three pack years ago thinking leggings are leggings, and by the second wash they'd gone see through at the knee and the waistband had started rolling down every time I sat on the sofa. A good pair costs more but the difference is in the waistband staying put and the fabric staying opaque wash after wash, which actually matters more with leggings than almost anything else you own, because thin fabric on tight fabric shows absolutely everything. For everyday wear and gentle exercise you can get away with a lighter weight. For running or anything with real movement, go for a pair with proper compression and a flat seam, because a seam that rubs on a forty minute run will ruin your week.
On sizing, go by your actual measurements rather than what you normally wear on top, because leggings brands are all over the place and true to size on one rail is a small miracle on the next. Check the length sitting down as well as standing, since a good ankle length standing up can creep up to a very different, much more interesting length once you're sat at a desk. And always check the waistband doesn't dig in when you sit, not just when you stand in front of the mirror looking pleased with yourself.
Reach for them on the days when you need to actually move, or when comfort has to win over everything else and you don't want to think about it twice.