The documentation for IOCTL_DISK_VERIFY says:
Verifies the specified extent on a fixed disk.
But what does that mean? When and why do I need to do this?
It means the device verifies the specified extent (range). It reads the data and if it can be read without errors the command succeeds otherwise it fails. No data is returned in either case.
There's generally no reason to use this IOCTL.