Logical grouping
Resource groups exist to help manage and organize your Azure resources. By placing resources of similar usage, type, or location in a resource group, you can provide order and organization to resources you create in Azure.
Logical grouping is the aspect that you're most interested in here, because there's a lot of disorder among our resources.
Life cycle
If you delete a resource group, all resources contained within it are also deleted. Organizing resources by life cycle can be useful in nonproduction environments, where you might try an experiment and then dispose of it.
Resource groups make it easy to remove a set of resources all at once.
Authorization
Resource groups are also a scope for applying role-based access control (RBAC) permissions.
By applying RBAC permissions to a resource group, you can ease administration and limit access to allow only what's needed.