I think it depends on the circumstances, physiology of the person and the fashion sense of the person wearing it.

If you want to create a waistline for yourself, you may wear a shirt tucked in. Traditional kilts hit at the natural waist. Not all of us benefit from foreshortening the torso in that way.

Wearing a sweater on the outside of the kilt lowers the visual waistline to the hips. It elongates the torso and helps hide bulges a little (depending on the fit and thickness of the sweater).

And, what the heck, let's split the difference. It is popular in casual fashion for men to tuck the front of a shirt or sweater in around the belt buckle and let it hang out the rest of the way around. It is a look better suited to thin men, in my opinion.