Tenant user's are enterprises which asks more flexibility in tenant hierarchy. Owner of tenant may want to create subtenants which mimics hierarchy of departments and/or projects. Number of levels can easily go beyond currently available two levels. There is support ticket 5385980 "How many levels of subtenant are supported?" which originated this idea.
To be taken into consideration is the ability for a customer to manage all their tenents and sub-tenants from a single perspective.
This requirement is raised from Turkcell. One of their enterprise customer wants to create their own tenant hieracrhy with Enterprise Tenant. With the current tenant-subtenant model, customer cannot isolate data of different branchs under one subtenant.