An Azure networking service that is used to provision private networks and optionally to connect to on-premises datacenters.
No, if you have an Azure App Service - standalone - it does not need a Virtual Network and subnet - the resources are managed in the backend - the same for Azure SQL. But if you are standing up an Azure SQL Managed Instance, or a service that is integrated with private link, the general applies:
When you create a managed instance, Azure allocates a number of virtual machines that depend on the tier you selected during provisioning. Because these virtual machines are associated with your subnet, they require IP addresses. To ensure high availability during regular operations and service maintenance, Azure might allocate more virtual machines. The number of required IP addresses in a subnet then becomes larger than the number of managed instances in that subnet.