Hold a ruched dress up on the hanger and it looks like nothing much, a crumpled tube with no shape to speak of. Put it on and it does something different for everyone, which is exactly why it's worth the fitting room faff. The gathering finds whatever you've got and skims it rather than clinging, so you get the smoothing effect of shapewear without the actual shapewear.
For everyday wear, keep the rest of the outfit quiet. A plain jacket, flat sandals in warmer months, boots and tights when it's colder, because the dress itself is already doing something visually busy and anything else ruched or patterned on top turns it into a muddle. I wear mine with a denim jacket for the school run and the same dress with heeled boots for an evening out, and nobody would clock it as the same item twice. Colour wise, the easiest ones to live with are solid and a bit deeper, bottle green, wine, navy, charcoal, because the ruching already creates shadow and texture, and a busy print fighting with that just looks fussy from a distance. If you want pattern, go small and dark rather than bold, a tiny ditsy print holds up far better under all that gathering than anything with a big repeat.
Fit is where ruched dresses either flatter you properly or sit wrong all day, and the mistake I made for years was sizing down because the fabric looks stretchy on the hanger. Go by your actual measurements, not the vanity sizing on the label, because ruched jersey will cling to whatever's too tight rather than skim it, and you'll spend the whole day tugging it off your middle. Check the gathering sits across the part of you it's meant to be smoothing, usually the stomach or hips, not an inch above or below, because ruching in the wrong place draws the eye straight to it. Watch the length too, a lot of these dresses ride up slightly when you sit down, so try sitting in the changing room before you buy, not just standing in front of the mirror.
As for when to wear it, ruched jersey is a three season dress rather than a summer one. It's too warm and clingy for proper heat, but it layers brilliantly under a coat from autumn through to early spring, and it's forgiving enough for anything from a work do to a dinner where you've eaten more than planned. Wash it on a cool cycle and let it drip dry, because tumble drying flattens the ruching and it never quite gathers the same way again.