The answer is that it explains the dynamics of the inflation process, how the economy evolves from one inflation rate to another, for example, in response to an increase in the rate of money growth. The dynamics of changes in inflation operate through excess demand in labor and/or product markets.
Thus the Phillips Curve indicates that, if the unemployment rate is maintained at a level below NAIRU, inflation increases over time, progressively and indefinitely. The initial source of an increase in inflation can be anything which produces excess demand in labor and output markets.