But do you have to look at the display to tell what the buttons and knobs are doing.
If you have, say, a HVAC fan speed knob with mechanical stops at the low and high end, and a detent, you never have to look at it. If you have an increase/decrease switch, you may need to look at the display to find out what you're doing. In a car, this means head-down time, eyes off the road.
I have a Black and Decker branded humidifier, which comes from "W Appliance Ltd", a licensee of the Black and Decker name. It's an ultrasonic humidifier with a 1.45 gallon tank and a big filter to remove dissolved solids, so it runs well on tap water. It's an effective humidifier.
This device is an example of how to botch the user interface for a very simple device. There's a big round display, about 12cm across. This has various dedicated icons and a central number display. Around it is a ring which displays a moving bar pattern when the device is running.
From left to right, we have five buttons. They're just touchable areas on the case, not actual pushable controls. The first button is On/Off, and, inevitably today, the same button does both functions. The display lights up when on.
The second button turns on a negative ion generator. This isn't an advertised feature, and it may not actually do anything. If this feature is on, a tiny icon illuminates on the display. This thing is down on the floor and you can't see the smaller icons without getting down on your knees. If you hold this button down for two seconds, the decorative bar pattern on the display is toned down, but not fully turned off.
The third button is fan speed. Available values are 1 to 3. Default is 2. 1 is useless, and 2 is mostly useless, because the water condenses on top of the unit rather than humidifying the room.
The fourth button sets the humidity. Values from 45% to 90% can be cycled through. There's one two-digit display, and it shows the humidity being set when the button has been pushed recently. Otherwise it shows the humidity being measured.
The fifth button sets a timer to turn the thing off after some number of hours.
When the water tank is empty, a tiny icon illuminates. The main display does not change or go dark.
The one actionable piece of info the device can give the user is barely visible.
Removing the water tank or turning the device off resets all settings to the defaults. So after each refill, the user must go through setup again.
There's an optional remote available, with the same five buttons.
All this thing needed was one big knob for setting the humidity, with an off position. Plus a nice big indicator light to indicate an empty tank. Instead, they designed a complex user interface that makes it worse.
This kind of mistake appears when UI people design button systems.