The reasons that you need a keyword for every page of your site are plentiful

1. more pages = more weight than a smaller site in googles eyes
2. dedicated pages to products link the search result directly to the product meaning
that the searcher does not have to then search your site from the landing page to find what they wanted.
3. the more keywords you target the more traffic you get
4. its easier to target a small amount of keywords with one page that a large amount

I build websites that get ranked number one for very low search volume keywords, yet they have massive amounts of keywords, meaning that the traffic is still big, plus if the site is nice enough then the bounce rate will stay low. that makes the site go up in the search engine for the more competetive keywords on that page.

You have to also get links back to your website pages.

Best thing to do is to build the site and then leave it a week. once all the pages are indexed, see where they land in the search results. then start at the highest results and link back to those pages from web 2.0 sites. Once you get a Number 1 for that word, move on to the next keyword, and so on.