The purpose of SSDT-PLUG is to allow the kernel's XCPM(XNU's CPU Power Management) to manage our CPU's power management. It's pretty self explanatory why you'd want this.
Note: SSDT-PLUG is only compatible with Intel's Haswell and newer CPUs, Sandy Bridge and Ivy Bridge will need to follow the ssdtPRgen method(in post-install) while AMD users should not use this(unless attempting to attach AGPM which is outside the scope of Dortania's guides)
For SSDT-PLUG, there are 3 methods you can choose from: