I can't find any information on documentation why connecting an Azure Devops Organisation with Microsoft Entra creates resources groups.
When I get the list of list of organizations backed by Microsoft Entra ID, I see some organizations.
However, no clue why I see RGs and Azure DevOps Organisation resource in Azure.
According to this doc,
Deleting the organization resource from the Azure portal removes billing for your organization and your organization returns to its free limits.
When you set up billing in Azure DevOps it will automatically create the resource group for you (VisualStudioOnline-xxx). If you delete this resource group, the billing will be removed for your Azure DevOps organization.