An unloaded transformer still draws a small current, the excitation current, which is what establishes the magnetic flux in the core and covers hysteresis and eddy-current losses. It usually sits between 0.5 % and 2 % of rated current.
The test applies reduced voltage to one winding with the other open-circuited and measures the resulting current on each phase and each tap. What matters is not so much the absolute value as the pattern: on a healthy three-phase transformer the two outer phases read similarly and the centre phase somewhat lower.
A departure from that pattern, or a change against the history, reveals magnetic circuit problems — shorted laminations, air gaps, loose clamping — as well as shorted turns and tap changer defects. It complements the turns ratio test: each catches what the other misses.