For the BIOS to have full control of all the features of the newer cpus, they all need to be enabled.More details please find it at http://www.techsupportforum.com/forums/f15/pros-and-cons-of-disabling-c-state-and-c1e-559253.html
Maybe this will help (taken from another post):
That was the case for older CPU's but the i3/i5/i7 benefit from both, SpeedStep is better for changing the multiplier/voltage but C State has additional benefits on the new Intel CPU's, instead of the whole CPU either being on/off/idle parts of the CPU can now be turned on/off or set to idle and this works in conjunction with intels Turbo Mode.
So basically they did do the same job but there are benefits to having both on when it comes to the new i3/i5/i7 CPU's.
you will want to set CxE Function to C6 to get these new benefits alongside having SpeedStep enabled (they can work independent of each other but its best to have both enabled, be warned though with newer EVGA BIOS's having CxE Function enabled will allow the higher Turbo Mode multipliers to kick in and could make your OC unstable, if this is the case disable CxE Function but you could keep SpeedStep enabled if it still works, on the X58 Classified the voltage part of SpeedStep does not work with a manually inserted Voltage, it does however still work on the E758 3X SLI board with a manually inserted vCore voltage, this is just due to the components used and how the boards are set-up due to the segments they are for, Classified being a primarily overclocking board when power saving features are secondary. There are still work around for the X58 Classifieds using the ECP, this should allow you to OC the CPU but use an AUTO voltage which would allow the voltage part of SpeedStep to work