Hey Scott,
as the Onefinity Controller is a stand-alone CNC Controller, the user interface on the display is mainly used to open the g-code file you want to mill, after you connected a USB drive with the file on it. You will also have to press “Play”/“Pause”/“Stop” to control g-code program run and press “Continue” on the popup windows when workpiece XYZ probing with the touch probe or on manual bit length probing on tool changes.
But you can get by completely without connecting a monitor directly to the Onefinity controller and skip buying the touch monitor, since the User Interface can be controlled completely from any remote computer. This even has the advantage that you can see the CAMotics.org 3D toolpath simulation that is not available on a monitor directly connected to the Onefinity controller, as the internal Raspberry Pi 3 has no 3D graphics acceleration package installed. In the case you use a remote computer, you need no USB drive since you can simply click the open file button and you will be able to select a file from your computer.
Image: The 3D toolpath simulation when viewed on a remote computer.