Any measurement that injects direct current into a winding — winding resistance being the most common — leaves the core magnetised. That residual magnetism distorts any later test that depends on the magnetic circuit and, if the transformer is energised in that state, produces an inrush current far larger than normal that can trip the protection.
Demagnetization removes the remanence by applying a sequence of alternating current, or DC of alternating polarity, with decreasing amplitude, tracing ever smaller hysteresis loops until the core is left in a neutral state. Modern test sets include it as an automatic function at the end of the measurement.
Skipping it is a frequent source of inconsistent results: inflated excitation currents, unstable turns ratio readings and impedance values that do not match the history. As a rule of thumb, demagnetize before considering the test campaign finished.