Guid acted as variable store header's signature and to specify the variable list entries put in the EFI system table.
Include/Guid/VariableFormat.h

GUID Information

GUID's Guid Name	GUID's Guid
gEfiVariableGuid	{ 0xddcf3616, 0x3275, 0x4164, { 0x98, 0xb6, 0xfe, 0x85, 0x70, 0x7f, 0xfe, 0x7d }}

Refer to Header File