When a piano has not been tuned on an annual basis the pitch will drop significantly. In such cases a standard tuning will not suffice to bring the instrument to A 440 with stability. A pitch raise is really 2 tunings in one with the purpose of bringing the instrument to an equilibrium at or near A440.
A pitch raise is much more labor intensive than a standard tuning. In the case of a piano having not been tuned in 20-30 years, extra care must be taken the prevent string breakage. In the case of extreme pitch raises a follow-up tuning within months is reccommended.