In World War I, the Scots kilt had become an industrial item, with the buckles and manufactured pleats and so on. It then acquired a khaki apron to protect the wool tartan material. Leggings were worn over the hose to the boot tops. Wool insulates when wet. Having worn a kilt in sodden, wet climate, it generally remains drier than wool pants, which get very muddy at the base of legs unless something like the leggings or roll puttees are worn, which most people don't, and surely were not by the vast majority of soldiers in the Civil War.
To put the wind up the Germans in WWI, the Scots in the kilt were the ticket.
To put the wind up the Italians in WWI, Bosnian Muslims in fezes were the ticket.
So the kilt, even shorn from its Scottish countryside origins and after the post-Culloden/ post-Jacobite outlawing of it, could be important for reasons of esprit de corps.
For anyone unaccustomed to wearing highland dress, the thick, heavy wool with lots of lanolin left in it can be agonizing after a short time of marching in it. Since no drawers are worn, it can chafe. If anything, a post-1820 industrial-era kilt of the typical thickness would be unbearably hot in the U.S. South. The one I have currently for wearing to social gatherings where dress attire is called for here in Texas is a relatively thin thing, and not what would be worn in Scotland. There is, of course, the present day abomination known as a "utilikilt" made of some sort of hard-wearing denim or whatever, replete with cargo pockets and so forth, but that is not a kilt, but a modern abomination.
The original highland dress doubled as the blanket for sleeping out of doors, or in an improvised shelter, and was therefore a very efficient garment for campaigning. That's why it continued in use for as long as it did, which is to say, until the '45... After that, it was mostly about esprit de corps.
I'd conclude by noting that the lowland Scots, like my late great-grandfather, who were in the Black Watch during WWI had literally never worn the kilt a day in their lives prior to military service.