As a publisher, I wouldn't want to make up lists of AdSense keywords for all 3,500 or so pages on my current site. That would be a huge project, and it would be a distraction from my job of writing and publishing editorial content. I'd much rather let Google's AdSense crawl my pages about rail travel and determine that rail-whatsit.com or train-widgets.com would be a perfect advertising match. Computers were invented, in part, to automate drudgework--and I can't think of a better term than "drudgework" for the process of assigning keywords to several thousand pages.
IMHO, the combination of AdWords and Google's search technology is what makes AdSense unique--and it's why I submitted an application earlier today.