Assume that you have an on-premise Hyper-V cluster that contains X number of virtual machines and the hosts are licensed under Microsoft Enterprise Agreement with Software Assurance. You are planning to replace the on-premise virtual machines with the Azure Virtual Machines. You need to minimize the compute costs of Azure virtual machines.
Enabling the Azure Hybrid Benefit for Azure Virtual Machines will allow you to use your on-premises Windows Server licenses and run VMs with reduced costs in Azure.
You can also use Azure Reserved Instances so that you can reserve virtual machines in advance and save more.
You can learn more about Hybrid Benefit by visiting the Azure docs page.