We expose to the right with a digital camera because of the way digital image sensors record information: they record more detailed information in bright areas than in dark areas, which means you see fine details and smooth ranges of tone and colour in bright areas and can see noise in dark areas.
In order to maximize the amount of recorded information, brighten your exposure without blowing out the highlight details. Exposing to the right means the histogram display is pushed to the right, in the brighter values.
This fine-tuning is key to achieve the best image quality.
