The piano tuners guild recommends tuning your piano every 6 months. Because bianos have metal strings and a metal frame, they can be easily affected by temperature, humidity and movement.
A fine tune is used when the piano is fairly close to 'concert pitch.' The strings can be tuned to perfect or nearly perfect and the piano will be in tune with other instruments.
A pitch raise is used when the piano is too flat to perform a fine tune. The strings will have to be tuned a lot, putting a lot of stress on the strings and frame. Because metal can stretch, a pitch raise will get the piano near concert pitch and sounding decent, but an additional tune or tunes will be required to make the piano perfect. Some pianos are so flat that they might require more than one pitch raise before a fine tune can be performed.