The PU AVG functiis on is used when you want to display the overal average order size for the entire firm through the different months of the year.

PU generally, are good to use when referencing other tables in a data model (DM). Especially due to the 1:N relationships established when constructing a DM. It is great for referencing any supplementary tables or multiple months too. Let me know if there was a specific query?