The finish has three different purposes. One: it protects the wood from dirty fingers. If you take your dirty hand and put it on a piece of 2 x 4 wood that has no finish on it, the dirt gets in it and you can't get it out. Your finish also protects the wood from increases and decreases in moistures such as high humidity. When the humidity rises and falls, wood expands and contracts. This happens when your door sticks to the frame when it rains outside. However, it's okay when it isn't raining. If wood keeps contracting and expanding very quickly, it breaks some joints. The third and least important thing is that it makes your pieces look pretty; but that's not the reason you need finished wood. It makes it look pretty, but its purpose is to protect the wood and keep it for generations.
