Windows 10 Why is there a +1 or -1 on a buffer or cast?

[ExylonFiber]

I cannot figure out why +1 or -1 is added to a cast or a buffer such as,

firmwareRequest = (PFIRMWARE_REQUEST_BLOCK)(srbControl + 1);

as listed on the MSDN page:

Upgrading Firmware for an NVMe Device - Windows drivers

Updates to the firmware on an NVMe storage device are issued to the miniport driver for that device.

docs.microsoft.com

I searched google and could not find the answer as to why this practice is implemented in programming pertaining to buffers and structures in particular.