SXA creates two media folders for new tenant:
What is the purpose of shared folder?
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sitecore/media library/Project/New tenant - this is media folder created for your project. In this example your project is called "New tenant"
sitecore/media library/Project/New tenant/shared media folder an be used when you have more then one site in the tenant. While using it you can share media across all of the pages within that tenant.