Why does the skin perspire?

To help us lose body heat. In a hot country or after vigorous exercise the body's temperature rises. Too much rise and you could die of heatstroke. So we perspire (well men perspire, ladies glow and animals sweat). This covers the skin in minute droplets of water which evaporate in the heat, absorbing latent heat of vapourisation from the body as it does. This cools us down. Latent heat of vapourisation is heat energy required for the molecules in a liquid to vibrate fast enough to escape from the confines of the liquid and become water vapour.