Arduino explains a little more : “the Arduino USB Host Shield is based on the MAX3421E, which is a USB peripheral/host controller containing the digital logic and analog circuitry necessary to ...

A USB host shield for the Arduino is needed to connect to an Android hand set. You’ll be able to send and receive strings via the terminal, with support for carriage return and life feed characters.

The idea early on was to leverage existing Arduino libraries to connect with a standard USB mouse, specifically, the hardware would take the form of an Arduino Mega 2560 with a USB Host Shield.