The Reader role is an Azure Resource Manager role that permits users to view storage account resources, but not modify them. It does not provide read permissions to data in Azure Storage, but only to account management resources. The Reader role is necessary so that users can navigate to blob containers in the Azure portal. For example, if you assign the Storage Blob Data Contributor role to user Mary at the level of a container named sample-container, then Mary is granted read, write, and delete access to all of the blobs in that container. However, if Mary wants to view a blob in the Azure portal, then the Storage Blob Data Contributor role by itself will not provide sufficient permissions to navigate through the portal to the blob in order to view it. The additional permissions are required to navigate through the portal and view the other resources that are visible there.
https://docs.microsoft.com/en-us/azure/storage/blobs/assign-azure-role-data-access?tabs=portal