1) To regulate body temperature: Our bodies sweat in order to regulate our temperatures - sweating cools the body by wetting the surface of the skin. Most people sweat when it's hot, or when they become too warm.
2) Protection: Sweating also helps to protect the skin as it is slightly acidic, and this helps protect skin from bacteria, as well as keeping the skin supple.
3) Feet contain a large number of sweat glands: The number of working sweat glands varies between parts of the body - and also between different people - but there are around two to four million sweat glands distributed all over our bodies. There 250,000 sweat glands in the feet alone - more than anywhere else on the body. When all the sweat glands are working at maximum capacity, we can produce up to three litres of sweat an hour.
4) Feet do a lot of heavy lifting: Our feet are under an enormous amount of pressure. Whether we’re running or walking, they take the strain of our entire bodies, and are all too often encased in shoes and socks that don’t allow the feet to breathe. No wonder we sometimes get clammy feet.
There is also a theory that sweaty feet would once have given us a genetic advantage. Some experts believe that sweat increases friction on bare feet, which would have helped our ancestors get more grip on the ground when running from predators - sweat being produced when our flight or fight mode kicks in.