Why the kilt is as it is today, I do not for certain know, I imagine it is a standardised, formalised garment developed from the belted plaid.
These images show several examples of how the plaid could be worn, with all of it hanging from the belt, near the floor, with it up around the shoulders for warmth, or gathered and over one shoulder to keep it out of the way. Also, the second fellow in the second picture has the plaid bound around his legs from the knee down, turning them into something like pants.





The plaid was essentialy a large rectangle of cloth, simple and utilitarian. it was an item for every occation. Remeber that in those days, travel was normaly done by foot, and would take several days. Often, this would mean sleeping under the stars, with the the huge amount of blanket to keep you warm. This (I think) is the reason for it being wool, as wool will keep you warm even when it is wet, thats why sheep dont die of hypothermia when it rains.
The reasons for the pleats are several fold, yes, they do add more layers and more insulation, but also, they keep the large amount of fabric well ordered, neat and easy to wear, rather then being all bundled up somewhere. The pleats also alow fantastic freedom of movement, alowing the bottom apature of the kilt to open just as far as you like.
Aprons, (again, i'm making assumptions, so feel free to correct me if you know differantly) as the plaid was a sheet of cloth, rather then a tube, it was important to have some overlap to protect your modesty, perhaps having pleats all the way around the front would be just too much fabric there to be comfortable.

When worn was one normaly imagines the plaid was worn (belted at the waist, hanging to the knees with the rest over a shoulder) it is easy to see where the modern kilt comes from. Simply remove the fabric above the belt, and sew the remainder to keep the pleats where they are, making a more practical and easier to don, garment.


PS this report is pure assumtion, based upon knowlege gathered here and there.

Sources;
Here, mcgurk 2007,
There, mcgurk, 1983-2007