Currently Todoist is rather limited with regards to insights about your completed tasks. Being able to archive them all in a spreadsheet opens up the possibility of analysis through pivot tables for instance.
Then
Add row to spreadsheet
This action will add a single row to the bottom of the first worksheet of a spreadsheet you specify. Note: a new spreadsheet is created after 2000 rows.
Discover more time saving integrations for Todoist and Google Sheets