I double checked my notes and the primary purpose is to speed up the search of transactions by either business partner or by products contained on the line items. If you didn't have this table it woudl be a nasty SQL statement to search for all business transactions with a particular business partner that contained a particular product at the line item level.
I don't have a coding example, but I will make a mental note that the second revision of my notes needs to expand that section and provide a few more examples.
Take care,
Stephen