What is the purpose of Shared document library in Sharepoint communication site and Microsoft Teams

tom_s1865 A Teams-connected SharePoint site is created automatically whenever you create a team in Microsoft Teams.

By default this SharePoint site will have a document library named Documents (Shared Documents) where you can add shared files which can be used/co-authored by your team members. This will be added to the Files tab in Microsoft Teams.

In Teams, the Files tab on each standard channel inside a team is connected to a folder in the parent site's default document library. The Files tab on each private and shared channel is connected to the default document library in the corresponding channel site. Whenever you add or update a file on the Files tab, you are accessing the SharePoint site.

Check below Microsoft documentations, these will help you with all your questions:

1. Overview of Teams and SharePoint integration 
2. Should I store my files in Microsoft Teams or in SharePoint? An understanding of behind the scenes 
3. Create a team site in SharePoint 

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