Is a cluster used separately from Availability Groups in practice?

Yes, in practice, a cluster can be used independently of Availability Groups (AG). A cluster provides a level of fault tolerance and resource management at the operating system level (for example, Windows Server Failover Cluster), while Availability Groups are a SQL Server feature for replication and high availability of databases.

Scenarios for using a cluster without AG:

• To ensure fault tolerance of applications or services not related to SQL Server.
• To support Failover Cluster Instances (FCI) of SQL Server, where the cluster manages shared storage and failover between nodes.

Thus, a cluster is a more general mechanism, and AG is specialized for databases. In some cases, only a cluster is used (for example, FCI), in others only AG (for replication without shared storage), and sometimes both together for comprehensive protection.