simple.... they optimize the inner pages of ur site which u submit as a deep link of the same site... with different keyword... & gives the inner pages more PAGE value/strength so that they also can directly attract SERP's..
Deep links helps - - To optimize your subpages - To index your subpages (if not indexed) - Gain PR for inner pages - Give traffic to subpages and more resources for visitors (if someone visits the details page )
Is it not easier to search a search engine to find what you are looking for than hope that the homepage you have found has what you are looking for somewhere hidden in that site.
If all the juice goes to the main page then what bout inner pages ??? where the content juice is more ??? So the competators site which has more strength in inner pages+content will index in SE & will overtake ur site...& in sites not all content is covered in main page... many sites have different NICHES so it has to be optimised with different keywords seperately...