Scotland is important for the UK, and the Royals wear tartan kilts to mirror their association with that land. When in Scotland, a portion of the more seasoned male individuals from the family wear kilts (not dresses). This is viewed as formal wear, particularly when worn with a coat and tie, and it's conventional, not restricted to Royals, yet to any man who has a place with a Scottish group with its own plaid. 

Their spouses will generally wear skirts or dresses in a similar plaid. In different circumstances, a portion of the female Royals wear plaid covers, skirts or dresses since plaid is a design look they like, and it's useful for cool environments. Frequently the plaid is woven from great fleece, which is warm. There are a few tartans explicit to specific families, and different tartans that are worn by anybody and accessible in apparel shops on the high road. The Royals aren't the ones in particular who wear plaid, in spite of the fact that it is something one sees most frequently in the UK, less so in the remainder of Europe.