USB boot En 1K resistor

Hi all,

I'm trying to understand what's the use of the resistor labelled as "USB boot En" in Arduino Mega 2560 and Uno schematics:

Designator: RN3D
http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-schematic.pdf

Designator: RN2D
http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf

Besides drawing some extra mA when pin is high, does it play a special role in the reset event from 16U2 when USB port is open?
What would happened if removed?

Thank you very much!