Interfaces have elements that govern something. For example: change the volume, the level of cooling / heating and the like. Often the scale of regulators is graded, numbered or, if the electronic version, the numbers are displayed.
And there is the effect that users adjust their subjective requirements to numbers. (I, for example, have a scale markup step of two on the sound control console. I put the volume a little more than four, the low frequencies are 2-4 :-)
By the way, I noticed that there is no designation in the BMW business CD (built-in player) when changing the volume. It seemed to me that it is very cool. For I put the sound as I want to hear; no matter how much this decibel. ')
All other signs of change, where it is easy to do without them, provoke schizophrenia at an early stage :-).