Pitch Raise and Piano Tuning: What's the Difference?
In short, a pitch raise is basically a pre-tuning: it gets a piano close enough to the right pitch so that it can be fine tuned accurately. Seasonal changes in temperature and humidity cause the wood parts to swell and contract, putting the piano out of tune. If a piano has not been tuned or serviced for several years, it will likely be far enough flat that it requires a pitch raise before the tuning.