they keep your feet warm.
Woollen Explorer™ socks are GREAT for camping events. They keep you warm (even when wet), keep you cool (even when dry), and are all round "comfy socks" because they're made out of 100% wool. They are also well padded, which works well if you are wearing unfamiliar boots, however they do look a little strange worn under a fancy girl's court dress.
"Over the knee" socks are as good an imitation of period hose as can be bought in the shops. I'm talking specifically about the ones that are mostly cotton, with only a little lycra, and have a nasty habit of falling down after the first few washes. Because that's what hose do unless you wear garters. Don't worry - garters are easily made from a piece of ribbon, or a quick piece of fingerloop braid. For Australians, Red Robbin™ brand socks are suitable - although they are expensive, they look good with garters and can be found in Myers stores. It's the slight bagginess of cotton over the knee socks that make them look more period - after all they didn't have stretch fabric in medieval times. In cold weather, however, medieval people would wear woollen hose (sometimes over a pair of linen hose for comfort), so they stayed really warm, even when wet. In cold weather over-the-knee cotton socks just get cold and damp, and have none of the nice insulating properties of medieval hose.
Stockings and pantyhose are often worn at feasts. They work well to keep the legs warm, but only look somewhat period, especially for earlier periods. The main points that make them look less period are: they are tight everywhere including the ankle, lycra often glistens differently to natural fibres, are often much thinner than period hose, and the height is often wrong - women's hose generally were only knee high, men's generally only to high on the thighs. Pantyhose chosen with these criteria in mind will generally look more period than others. Men beware - pantyhose show much more of your curves than period hose would have. Thin leggings, stretch stirrup pants and other tight-fitting tracksuit pants can also be worn to good effect if the person buying them is familiar with the legwear of period they are trying to imitate (look at some paintings).