Skin on toes and fingers becomes wrinkled in water to allow people to grip objects in wet conditions. This is an involuntary reaction that cannot be controlled consciously.
When fingers and toes become wrinkled, the wrinkles act as a form of tread, like on tires. The tread provides extra grip on potentially slippery objects. This phenomenon was discovered in a 2011 study conducted by Mark Changizi in Boise, Idaho. When measuring how fast people could move wet objects, such as marbles, the people with wrinkles in their fingers were able to move them much faster than the control group that had normal dry fingers.