"The problem is that Window’s default power profiles are configured far too aggressively when it comes to core parking, especially on workstations. Their interest was in conserving energy, even if this meant marginally decreasing performance. A number of complex parameters control when a core should be parked, and Microsoft tuned heavily towards power savings."
This is a somewhat complex tool probably caused by the fact that processors are only getting more complex. However, what the program actually does and who it helps seems to take a bit of a back seat on the website to tech background and other details. The site seems to indicate that AMD processors benefit most from this tweak.
Based on the website, ParkControl improves latency in situations where your processor is idling and then needs to do something that stresses multiple cores.
Modern processors can completely turn off cores when only one core is needed. It comes at a cost, though: turning the core back on takes some time, up to ~30 milliseconds.
With ParkControl, you can tell Windows to always keep more than one core enabled, even in idle state.