There are several reasons to properly maintain a wood cutting board.
Repeated exposure to food and then subsequent soap and water will remove the natural oils from the wood causing it to dry, shrink and prematurely age. Regularly oiling the board restores the oil in the wood and keeps the wood young.
The primary reason to use a wood cutting board is to protect the edge of your knife blades. Properly cared for, the wood board will not dull the knife edge as fast as other potential surfaces you may choose to cut on (like your granite or marble counter which is not good). If the cutting board is dry (without oil in the wood) it is also very hard. So oiling your cutting board regularly will help maintain the edge on your knives.
Finally, wood is porous, meaning it absorbs liquids. If a cutting board is properly oiled it will not absorb water and food juices as you use t to prepare food. Residue left on the surface is easily washed off and the underlying wood is kept from retaining food particles by the saturation of oil. A dry cutting board can absorb food juices and particles which can then spoil and cross contaminate foods during subsequent uses.
Use any food grade (white) mineral oil or better, buy a product designed and label for cutting board maintenance.