Dont take faith, when you find things like this you rather should read documentation and find out more. Its a genuine occasion to become better, by all means do not let these moments pass by because of "faith". Faith is bad and leads to bad things.
If you open the mega sources, and find the regulator name, and then check NCP1117 datasheet and read at start of page 10 under "protection diodes". And to convince that is true, feed the usb with 5v and check the voltage on the open Vin line, should be 5v perhaps minus a small dropout. If the ramps D1 would not block this voltage, it would reach back into whatever its connected at that pin. Basically D1 keeps the 5v flowing out of the arduino back through the Vin pin, which in our case leads inside atx psu.
If you open the mega sources, and find the regulator name, and then check NCP1117 datasheet and read at start of page 10 under "protection diodes". And to convince that is true, feed the usb with 5v and check the voltage on the open Vin line, should be 5v perhaps minus a small dropout. If the ramps D1 would not block this voltage, it would reach back into whatever its connected at that pin. Basically D1 keeps the 5v flowing out of the arduino back through the Vin pin, which in our case leads inside atx psu.