The idea is to protect pin 9 of U4 from high voltage or negative voltage on VBUS_USB.
VBUS USB goes to something outside of the circuit board. Since it is a USB C power source, it could legitimately have up to 19V on it.
Bad things can happen, though, so that you could end up with more than 19V on that pin - or a negative voltage.
R33 and R43 form a voltage divider to reduce the expected 19V to a maximum of 1.9V.
If the voltage at the junction of R33 and R34 goes above about 3.4V, then one of the diodes clamps the voltage to 3V3.
If the voltage at the junction of R33 and R34 goes below about -0.1V, then the other diode clamps the voltage to 0V.
Both protect the input of U4.
Since VBUS_USB has to go through R33, the current through the diodes will be very low - no danger to the power rails.
