I think it is northbridge/southbridge drivers and all of the sub components therein. PCIe controller, PCI controller, USB2.0, SATA controller I think if non-raid etc etc.
It's always the first thing I install after a windows install.. so I guess I have never noticed a difference with or without since I don't really run without it.
It's the chipset drivers for intel chipset motherboards, these drivers are for the hardware devices on the motherboard and must be loaded just after the OS install.