xcoder - 11:01 am on Nov 1, 2012 (gmt 0)
Search phrases tell them more, but even there they can't always easily tell whether "widgets" means I want to find out about widgets or whether I want to buy one.
They have lists of "buy intent" keywords and key-phrases across every imaginable market known to man, put together by you guessed it, adwords advertisers. Who knows better then those advertisers what is a "buy intent" key string and what is not. Especially the top converting advertisers, silly enough to have their Goog analytics tied down to their ad campaigns for conversion tracking.
Guess what google later does with all this information...they use it to filter and distil "buyers" traffic. Guaranteed!