StarRC extraction types vs process corners from foundry
Corner simulation is usually done for dedicated devices like resistors and capacitors that are used in the design. Both types of devices usually have quite a large variation on the order +/- 20% and therefore corner simulation gives valuable information.

Parasitic extraction results in additional (parasitic) devices because interconnect has both resistance and capacitance. These parasitics are neither very accurate nor well controlled. Therefore the design should be such, that it does not degrade too much in the presence of parasitics.

I'm not sure if the corner models you have actually cover the variation of the parasitics as well. Anyway, just because you chose a certain corner (like RCbest) does not require you to actually have these components in your design. It just means that the simulation models are included.