Purchases and purchased line items


I have a question about purchases and purchased line items. I have read the documentation, but I am having a trouble fully understanding these modules. 

What exactly is the difference between purchases and purchased line items?

Are purchases equivalent to the opportunity, and PLI to revenue line items? 

Is there only one Purchase created and new purchased line item is assigned to it?

How do these two work with renewals?

What is the purpose and best use of these two modules?

Thank you!