reducing the effect of a parasitic capacitor!
Capacitive shields only work when noise is being coupled into you signal externally via electrostatic means. You have some rapidly changing high(ish) voltage (HF/AC) somewhere that has a little bit of capacitive coupling with your signal of interest, the shield... well acts as a shield, it provides a much lower impedance return path for the noise and because the shield is grounded (or connected to some low impedance source) it has very little AC voltage across it, because it has so little AC voltage across it, it doesn't produce strong electrostatic fields and so doesn't couple strongly with your signal.