Generally multimeter readings are far more accurate than in-BIOS or in-OS readouts, as it can take factors like vdrop and vdroop into consideration to show the actual voltages being applied to the silicon.
It also allows up to two of the CPU voltage, vtt, memory and P55 chispet voltage points to be checked simultaneously because of the access to separate grounding.
This YouTube clip shows the V-Kit in action:
Source: Bit Tech