Google has a link to shopping on its search engine where it lists various products. Sort of product listings and comparisons.
So, if you want to get customers through Google, you need to list your products there.
But how do you get your products listed there.
This is where Google Base feeder (earlier Froogle) and now Google Merchant feeder comes in.
Using Google Base feeder or Google Merchant Feeder, you can prepare a file in xml format (which is the standard accepted by Google) and upload to the Google Merchant centre.
Useful module as it gets your products into Google listings. Sort of better than SEO.