Flaps, or more properly “trailing edge high lift devices,” have two functions—they increase drag and enable the wing to sustain lift at a lower speed.
The purpose of increasing drag is not so much to steepen the approach as to require you to use some power to maintain a desired approach angle. If you carry a moderate amount of power during the approach, you can adjust the aim point with small throttle movements. Airplanes that approach at idle power, as ones with low-wing loadings like old taildraggers do, have to use the somewhat less delicate forward slip to steepen the approach.
