Power Factor Correction (PFC) is needed to control the input current to the bridge rectifier, effectively the current that the whole setup draws from the mains. The input side of the board is a rectified voltage source with sine wave voltage input and capacitive filtering that draws current pulses with high peak amplitude which means a low power factor. With the PFC running however, we can control the input current to have the same shape and phase as the input voltage (sinusoidal usually) which puts less stress on the mains (high power factor and low total harmonic distortion).