I suspect it is to make a neat outline, removing the bulk of the material and holding it close to the body rather than the sack tied in the middle look of a casually donned great kilt.

It is similar to the alteration in the countryman's smock to make it into the modern shirt.

These days we see lots of people wearing baggy clothing, saggy trousers, so we think little of it, but after Beau Brummel the manly outline was decidedly a smoothly tailored one. Victorian Englishmen even wore corsets to narrow their waists.

The unstructured great kilt simply would not do.

Anne the Pleater