UI stands for User Interface. That could be anything from nothing, over a number of buttons, switches and leds to a touch screen or even a PC or a cell phone.
And while you often do not need many, if any, UI elements to start a process (i.e. insert USB stick, everything else is done automatically - what do I need an UI for ?),
You might to have some status information, like how much print time remaining.
and you will be needing one as soon as problems occur. The user would want to get an indication of the problem - wrong file format or no file ? Is the motor blocked ? and so on.
Than again you could connect a PC for diagnostics, so the device itself does not need any kind of status display, but that somehow defies the stand alone approach - you see it's a can of worms, many ways to solve (and create
The point is the possibilities are endless and when you exactly know what you want you can choose the one that you think fits best.
Knowing exactly what you want and expressing it is a serious problem, getting it understood is a even more serious problem, especially if the one expressing and the one listening have different backgrounds/concepts/ideas how things work i.e. programmer - jeweler.