Our hands are pretty amazing parts of our bodies. Think about all the things they can do! A person who knows sign language can count every number imaginable just by moving his or her hand in different shapes. Our hands can bend, they can stretch, they can wiggle, and they can wave! To be able to do that, the skin that covers our hands needs to be able to adjust to the changing shapes.
That's where those lines come in handy. They're technically called "flexion creases," and many of them are on our hands from before we are born! When our hands bend, the creases help fold the skin. Otherwise, whenever we made a fist, we'd just have this blob of loose skin hanging out under our fingers. Yuck!
Take a look at all those lines on your palms and fingers. Each one is there because it is at a place where our body bends.