Why A point-contact diode is made by pressing a fine metal wire against a semiconductor surface, giving an extremely small junction area. That means very low junction capacitance, so the diode can rectify at VHF, UHF and microwave frequencies where an ordinary PN junction would simply pass the signal through its own capacitance. That speed makes it the classic detector for demodulating or sensing RF, as in the old 'cat's whisker' crystal set and in modern RF probes and power meters.
Watch out Using it as a high-voltage rectifier is the trap: the tiny contact area gives a low reverse breakdown voltage and very little current handling, so it fails in power rectifier service. Constant voltage is a Zener's job, and constant current is what a current-regulator diode does.
Tiny contact, tiny capacitance, fast enough for RF: point-contact means detector, not power rectifier.