Your skin accounts for over 15% of your body mass and is your first line of defense against the elements. One of the primary elements your skin defends against is solar radiation. Solar radiation causes skin to darken as it becomes damaged and ultimately burns the skin. This damage never goes away, but accumulates over time.
Your body develops moles which are concentrations of melanin, a skin pigment which helps protect against radiation (among other things). Moles on the body should be watched so as to make sure they do not become cancerous.