in general, a capacitor is seen as a storage component for electric energy. but this is only one capacitor function. a capacitor can also act as an ac resistor. especially aluminum electrolytic capacitors in many applications are used as decoupling capacitors to filter or bypass undesired biased ac frequencies to the ground or for capacitive coupling of audio ac signals. then the dielectric is used only for blocking dc. for such applications the ac resistance, the impedance, is as important as the capacitance value.