While modelling a transformer we must account for the lossses and leakage flux in the equivalent circuit. Core losses are modelled by Resistor.
But how come inductors are used to model the leakage flux?
You have a perfect transformer that does not radiate any magnetic flux and to represent the actual leakage flux you add an extra inductor in series with it that does. This is called the leakage inductance. It can be referred to the primary or the secondary as more useful,I reckon secondary is better then it can be put in series with the load.
Frank
If you are interested in a physical explanation, you may be interested in this profound Dixon paper from the Unitrode/TI magnetics design handbook Deriving the equivalent electrical circuit from the magnetic device physical properties**broken link removed**