Purpose of Diodes in PIC32MM USB Curiosity Dev Board

Hi all,
 
I am designing a simple USB bus-powered application and was looking at the PIC32MM USB Curiosity Dev Board schematics as a starting point. There isn't much that a PIC32MM needs to run off of USB (basically just an external 3.3V regulator and some resistors/caps) but I wasn't quite sure what the purpose of these diodes (D2 and D3) is. In my application I only need one usb port (to communicate with the host) but the dev board has two (see attached images).
 
Schematic can be found here: https://ww1.microchip.com...ck%20Start%20Guide.pdf
VBUS_COMMON only connects to these two points/single node.
 
At first I thought that they seem to be protection diodes to keep voltage spikes on the board from flowing back to the host, but wouldn't you usually want to protect your device from voltage spikes? By putting a fast zener between 5V and gnd? The host computer already has ESD protection implemented, or it should.
 
My other thought is that they are there to allow the user to plug both of these USB ports in at the same time. This makes more sense to me, but I wanted to confirm this before I omit the diode(s) in my circuit.
 
I've been looking around for USB best-practice circuit design tips but haven't found anything quite like these diodes.