Why is a loading coil often used with an HF mobile vertical antenna?

HF mobile vertical antennas are almost always physically shorter than a natural quarter-wavelength for the band they operate on. Any vertical antenna that is shorter than a quarter-wave will present a complex feedpoint impedance with a highly capacitive reactance. A loading coil is simply an inductor placed in series with the radiating element. Because inductive reactance mathematically cancels out capacitive reactance, the coil effectively 'tunes out' the antenna's capacitance, bringing the antenna system into resonance. It does not lower losses; in fact, the resistance of the wire in the loading coil actually introduces minor losses and lowers the overall efficiency.