In the 19th century, Sir Walter Scott ‘re-invented’ the kilt in its modern form and from that point on it began to be recognised as our National Dress (dress as in costume, not skirt) worn mostly on ceremonial / formal occasions. Now, more and more, it is worn as a symbol of Scottishness, for instance by fans attending Scotland football / rugby matches. It is not everyday wear for most Scots.
The wearing of kilts goes back along time. The Scottish army wore kilts out of mere convience. In battle it is alot easier to relieve yourself if you don’t have a whole bunch of buttons and a belt to undo. Its also why traditional kilt wearing does not involve the use of underwear.