I challenge the idea of "body capacitance" here. Where is this capacitance "in the circuit" ? Between "Ground" and ????
The significant effect is that your body is a noise source when it is electrically floating.
When you touch "Ground" (typically by being in contact with the grounded metal parts/strings on a guitar) your body is held at the same potential and ceases to be a source of noise. You could also touch any other grounded metal for a similar effect - equipment rack / metal chassis etc. And it can be useful to arrange such a connection to your body to avoid noise when you might lose connection via the guitar due to playing technique.tapewound or coated strings etc.
I have had people doubt, or full out deny the body as a noise source. I tell them to touch the tip of a 'scope probe and observe the distorted 50Hz waveform on the trace. (60Hz if that is your mains frequency but it doesn't sound as good
).
And if you get a periodic noise at one second intervals - it's your watch (yep - been there
)