Parasitic Capacitance Diodes
Often you want to use a diode to block currents flowing in a certain direction. But the parasitic capacitor will pass high-frequency current signals, regardless of phase.

Often you want to use a  diode to provide a high-impedance in normal operation (while providing a path to divert unwanted signals like ESD strikes). The parasitic capacitance causes the diode to pass high-frequency signals rather than block them.