The formula for the voltage divider's output
\$V_{out} = V_{in}\cdot \frac{R2}{R1+R2}\$
only applies if you're not drawing any current of the output node. This because the formula above presumes that the same current flows through both the resistors R1 and R2.
In other words, to have a stable output voltage Vout
you either have to take the required output current into your calculation or it has to be small compared to the current through R2. For that reason you cannot use a simple voltage divider to power e.g. a microcontroller.
A potential without a current is a signal, either an analogue one or a digital one. In this case you'd route it on a high impedance input pin of a device and therefore the application with the voltage divider works (as no current is drawn). You'd do this e.g. to lower the logic level of an input signal from 5V to 3.3V.