I’ve long thought that our fingers and toes get wrinkly after spending time in the water due to some sort of osmosis effect. It turns out that is wrong.
In 2011 researchers proposed that our fingers get wrinkly after time in the water because it helps us pick up and grip wet objects and wrinkly toes increase our footing on wet surfaces. This theory has been confirmed in many experiments whereby subjects with wrinkly fingers are able to more quickly and easily pick up wet objects than those with normal fingers. So, the wrinkles in our fingers are actually rain treads.
Source: https://www.scientificamerican.com/article/why-do-our-fingers-and-toes-wrinkle-during-a-bath/
Actually, the process is osmosis, the evolution reason might be for better grip. This process happens in reverse, when salt water marine mammals (like dolphins) spend too long a time in fresh water, resulting ultimately in death.