Works best over a dress with an a-line or full skirt. The bottom of the cardigan will ideally hit at the horizontal seam line between the bodice and the skirt, maintaining the shape of the dress. Although a longer cardigan may look fine with the dress when you are looking straight on in the mirror, if you turn to look at your back you will notice that the proportions are off. The top of the skirt gets smushed down by the cardigan and then poofs out dramatically directly below the cardigan which adds extra fabric (and emphasis) on your rear end instead of highlighting your smaller waist.
These cardigans hit at or around your hips and they are great for keeping the look of long legs. Pair with wide leg or bootcut trousers and straight skirts. (An a-line skirt should follow the same rules as an a-line dress.) You want to avoid a long cardigan with these types of bottoms because they interfere with the line of the leg. The eye starts measuring the length of the leg from the bottom of the cardigan so if you have yours pulled down past your butt, you have not only lost several inches of leg, but added those inches to your torso. That creates a disproportionate body shape.
The long length works well with skinny jeans (and slim cut pants in general), especially when the pants are tucked into riding boots. The pant in boot phenomenon means that there is no exposed hem of the pant to visually bisect the leg. Since riding boots come up fairly high, we see the boot as a continuation of the leg, rather than it marking the end of the leg. This gives a longer leg line to work with which means you can add extra length to your torso without creating a wonky silhouette.