Without evaporative cooling people would overheat and collapse when moving around in hot seasons, and bodies of water need to be able to release heat in some way, they can only absorb so much. That heat is released with the water vapour that becomes cloud becomes rain feeds plants etc and the heat energy -> the movement of water particles, but also, the heat is released from the lake/ocean so that a normal temperature and ecosystem can be maintained, in order for marine life to continue in a somewhat constant cycle.