A digital multimeter is a combination instrument that can measure several different electrical quantities, so it must first be told which quantity and range the user wants before it can display a meaningful value.

The rotary selector switch performs this choice: turning it points at a labelled position such as DC volts, AC volts, current or resistance.

Internally, moving the selector connects the appropriate measuring circuit, shunt or reference into use and sets the range, so the meter interprets the input correctly.

Adjusting display brightness is not a multimeter function of the main dial, and on most meters brightness is fixed or set by a small separate control.

Resetting the instrument is done by power cycling or a dedicated button, not by the measurement selector.

Storing readings, where available, is handled by a HOLD or MEM key, again separate from the function dial.

So the selector switch's role is specifically to choose what type of measurement the meter makes.

Hence, the answer is Choosing measurement type.