When reservations are scoped only to a single subscription, any unused capacity cannot be applied to matching resources in other subscriptions within the same tenant. This leads to underutilization of the committed reservation and continued on-demand charges in other parts of the organization. Enabling Shared scope allows all eligible subscriptions to consume the reservation benefit, improving utilization and reducing overall spend. This is particularly impactful in environments with decentralized provisioning, such as across dev/test/prod subscriptions or multiple business units.

As workloads evolve, Azure Reserved Instances (RIs) may no longer align with actual usage — due to refactoring, region changes, autoscaling, or instance-type drift. When this happens, the committed usage goes unused, while new workloads run on non-covered SKUs, resulting in both underutilized reservations and full-price on-demand charges elsewhere.

The root inefficiency is architectural or operational drift away from what was originally committed — often due to team autonomy, poor RI governance, or legacy commitments. This leads to silent waste unless workloads are re-aligned to match existing reservations.