The most valuable use of AdWords for SEO is to research keywords. Keywords are the basis of any SEO campaign and even if you are an expert in your niche, you should always research keywords simply because users frequently search for quite unexpected keywords and keyphrases you as an expert will never think of. Needless to say, what matters most for high rankings is which keywords your target audience is searching for, not which keywords you as an expert think are most popular in a particular niche.