If your parents tell you to take a bath, then that's the reason why. But there is no survival reason to take a bath. People can go for months without taking a bath and they survive fine.
But, when they don't take a bath, people sometimes smell pretty bad. Taking a bath gets rid of the smell. And your skin gets dirty, just like a car gets dirty if you don't wash it. Taking a bath lets you clean your skin. If you have a cut or a scrape, cleaning the skin will help it heal faster because it gets rid of germs.