Wood is a porous material. It contains countless holes of various sizes. These holes can collect dirt and grime from handling, atmospheric conditions, contaminants, and food. Grime in the wood pores is unattractive and can be a health hazard, providing a breeding ground for bacteria. A finish seals the porous surface to help prevent soiling and make it easier to clean.
Wood is not only porous; it's hygroscopic – it absorbs and releases moisture. Wood responds to changes in the level of moisture around it. The moisture is absorbed by the wood, causing swelling. The opposite occurs when you put wood in a dry climate – it shrinks. The surface of wood responds more readily than the core. Wood mainly swells and shrinks across the grain. This causes stresses in the wood which can result in splitting, checking, and warping.
By using finishes, you can decorate wood with color, texture, and sheen.