You can have more efficient backup and recovery operations, reducing downtime in the event of data loss or system failure, by frequently backing up your data disks. If the data disk experiences an issue, it's easier to recover since it's separate from the OS disk.
You can expand your storage capacity by adding or expanding data disks without affecting the OS disk. Only data disks support live resize, you can't increase the size of OS disks without stopping the VM.
Separating the OS from your applications and data onto different disks helps ensure that disks IOPs from the operating system and the application data don't interfere with each other. This can enhance overall system performance, and prevent contention issues that might happen when both the OS and applications are competing for disk resources.
If there's a system failure or corruption occurs on the OS disk, you can reimage or reinstall the operating system without affecting the data disk. This reduces the risk of data loss and makes it easier when troubleshooting and for maintenance procedures.
You can apply separate access controls and permissions to the OS disk and the data disk. This separation limits access to critical system files on the OS disk, mitigating potential security risks.