A fixed powder skirt is permanently built into the jacket's hem, creating a seal around your waist that blocks snow from sneaking in from below. The big advantage is it's always there when you need it—no fumbling with zippers or forgetting to attach it before a deep run.
This makes fixed skirts a great choice if you regularly ride in deep powder or spend most of your season chasing storms. If you fall often, that reliable seal can save you from a miserable, snow-filled waistline. The downside is you can't remove it on warmer days or groomer laps, so it can feel constricting, bunched up, or overly warm when conditions are milder.
Go with a fixed powder skirt if deep snow is your normal riding environment. If you mix powder days with spring laps or resort groomers, a detachable skirt gives you more flexibility.