Q: Why is it important to always perform quick format in my Azure virtual machines for data disks?
A: All virtual hard disks in Azure are stored as fixed-size VHDs, which means the space is all allocated at the time of creation. However, behind the scenes sparse storage is used, which means only data actually written is stored and therefore what you're charged for. This is why it's a best practice to create all data disks in Azure as 1TB, because it's difficult to grow them and it costs you no more than creating a 1GB VHD. For example, if you have a 1TB VHD and write 100MB of data to it, then you only pay Azure storage for 100MB.
If you perform a full format, the format process writes 0s to every block of the disk. A 0 is data, which means the entire 1TB would be stored on disk and you would pay for 1TB of storage even though it's empty. Always perform a quick format to avoid writing data to the disk and to minimize your costs.