Whenever the rotor disc is not at right angles to the rotor mast, the mast receives a cyclical bending moment which causes stress. By tilting the mast forward, the maximum value of that stress is reduced - you get a bit of stress in the cruise and a bit of stress in the hover. If the mast were upright you would get less stress in the hover but a whole lot more in the cruise (of course in the cruise the disc is tilted forward so that the rotor thrust is tilted forward to overcome fuselage drag).
HC