A diode OR is used to isolate two or more potential power sources. The circuit behind the diode OR draws power from the highest voltage input source.
This is commonly used for selecting between multiple power sources. For instance, a device with a 9v battery pack and a 12v power brick will be powered by the battery unless the power brick is plugged in. It can also be used to draw power from multiple independent batteries in a balanced fashion, while still allowing independent balance charging of the cells.