Site collection documents is a system library that was created by the Publishing Resources feature to store documents that are used throughout the site collection.
The publishing feature does the following three things:
1. Create the necessary lists need for publishing
2. Sets the necessary web properties needed for publishing
3. Associates the various global items (like content types and workflows) with lists
The publishing feature creates the following items:
In this, the Documents library is a system library used for storing documents that are used on pages in this site.