Why do i need 2017’s AGI number (when its 2019) when i efile and 2018’s AGI when i file? and what do either of those things have anything to do with me paying the IRS?

Even though it is 2019, you are filing your 2018 tax return, so the 2017 is the last year of tax returns that were filed.

The IRS requires this number in order to e-file as a means of protecting you from identity theft. 

You do not need to provide this information if you are filing by mail.