Why Monitor a Microsoft Azure Subscription?
A subscription is the gateway to resources deployed on and services offered by the Azure cloud.
If any service instance is acting up - for eg., if a mission-critical application deployed on the cloud is inaccessible, or if an Azure VM is hogging resources, or if a specific Azure Storage account is running out of storage space, or if auto-scaling issues are noticed in specific virtual machine scale sets etc., - administrators would want to rapidly zero-in on the problematic instance and also the subscription using it. From an administrator's standpoint, such subscription-level insights ease problem identification and consequently, troubleshooting. As a result, problems will be resolved quickly, thereby ensuring the high uptime and good health of critical cloud services.
Also, since resources are grouped per subscription, cloud service providers often prefer to track service usage, measure resource consumption, and compute usage costs of their customers at the individual subscription-level only. This simplifies billing and resource management.
With per-subscription billing, cloud consumers too find it easier to monitor and manage their cloud spend.
In short, for obtaining focused metrics on service health, resource usage and allocations, and overall service performance, which will help all the stakeholders of a cloud organization take effective performance / scaling / costing decisions, individual Azure subscriptions should be monitored.