Increased Vr procedures are simply a matter of using the excess performance available when operating at less than Performance Limited TOW to mitigate against the possibility of Windshear.
The philosophy is that, if you can rotate at a certain speed and comply with the field length and climb gradient at a limiting weight, you will CERTAINLY also meet the requirements if you rotate at that speed at a REDUCED weight.
If you were to use all three v speeds and thrust setting for RTOW at a lower weight than they were calculated for, you would still achieve an excess of performance.
BUT your BEST performance for your current weight engine out will be achieved at v2-v2+15 for your ACTUAL weight.
SO- take the extra energy from the increased Vr into the air, as an extra buffer against windshear after lift-off.
HOWEVER if you encounter windshear on the runway OR have an engine-out, revert to the best performance for you ACTUAL weight- the V2 you have calculated