Why impedance of Transformer is given in %age?

It is really very practical and helpful. When a transformer is tested for impedance, the secondary is short circuited and a variable sine wave voltage applied to the primary, increasing from zero voltage until rated secondary current is got. The primary voltage applied to get rated current (with secondary shorted) is the "impedance voltage". If it is given in "% of rated voltage" (commonly referred to as %!) one can compare transformers of different kVA rating and primary voltage with respect to regulation or short circuit current without working in volts and ohms. For example, if a transfo has 10% impedance, you will know it will give rated secondary current, on (secondary) short-circuit test, with 10% rated primary voltage and hence ten times rated secondary current at rated (100%) primary voltage. You can also work out its approximate load regulation in % without dealing in the rated voltages, currents and ohms. When the short circuit test is done at rated current, the power (watts) going in is measured with a wattmeter and the power factor measured. This gives the resistive losses of the transformer at rated current for most practical purposes. The power factor gives a key to the resistance and inductive reactance components of the impedance voltage - which enable regulation at different load power factors to be calculated. The other basic proving test of a transformer is watts input at rated primary voltage with secondary open circuit. This gives one the magnetising losses at rated voltage. The international standards e.g. IEC 76 give the accepted practices for warm-up time, correction for winding temperature during test (the standard rated air temperature is 40 Celsius) etc. You should also look-up "per unit" system, this is what the power system analysis "gurus" use. It is even more mathematically simple and rigorous and useful than %!