Diode ORing is iseful if you need to combine signals or (more important) if you want to supply a circuit with different power sources (e.g. battery and external power supply or USB…).

If you want to supply something with USB and external supply voltage, you can do something like this:

The voltage regulator can be supplied by two sources. If no external power supply is connected D4 is conducting and powering up the voltage regulator. If you connect an external power source D5 becomes conducting and is pushing a higher voltage than USB, so D4 shuts off. Now poer only comes from the external source and USB is protected by the diode. Keep in mind, that GND is still connected.

If you want to supply something with battery and power supply, this might be something for you:

By using a P-MOSFET in the path for the battery, it only is switched on when no external power supply is connected. If it is, T4 is switched off, saving power from the battery. D3 conducts power to the regulator. If the external source (V+) is shut off, T4 switches on and the circuit is powered by the battery.