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.