Hi TxSteve,
Welcome to community and thank you for sharing the experience with us.
I went through your post carefully and I understand you want to know what the Description column is used for in a document library.
When I follow your steps to add the Description column by editing the current view of the library, I find the Description column is read-only too.
Honestly I don't find any official document mentioning about the introduction for the Description column. Very sorry about it.
In my opinion, it can be used to add description for each document (item) in the document library.
You may follow steps below to make the Description column editable:
1.Click the arrow next to the Description column and click Column settings > Edit.
The Edit column pane will show up.
2.Click More options and turn on Require that this column contains information. Click Save.
3.Then when you go to edit in grid view, the Description column will be editable.
Note: when you try to edit the Description column, make sure the document is not opening, otherwise you won't be able to save the changes.
Have a nice day and stay safe.
Best Regards,
Tina