Why was productive and unproductive labour considered to be at the core of Smiths arguments in WoN

In Book Two Smith provides an in-depth analysis of productive and unproductive labour, in book one he spends a lot of time in the division labour. But why was it that many economists considered that the analysis of productive vs unproductive labour was at the core of his overall arguments. And if this is indeed was the case what would you briefly his core arguments were in WoN.