Primer is designed specifically to bond to difficult surfaces; it gives the paint an even surface to which it can bond. According to About.com, priming a wood surface prior to painting it is essential for a professional-looking paint job.
Priming a wood surface eliminates the problem of flashing, which occurs when different parts of the finished job look as if they have been coated in different paints. With flashing, one area may look glossy and well-covered, while a spot a few inches away may look thin and dull. Priming also helps cover up any imperfections in the wood underneath and gives the finished job a full, even look.