Before getting to usage and understanding of workings of a transformer, it is important to note what is it and how it helps on a regular basis.
A transformer is specifically that electronic device which helps in transferring of electrical energy via electromagnetic induction between more than single circuits.
Here, to ease the understanding process, ideal transformer is taken as an example. An ideal transformer has negligible winding resistance power with no loss in the core value. With primary resistance coming down to minimal amounts, effective resistance is also zero and resulting in an extensively reactive circuit. Also, lack of energy loss in circuits adds up to this circuit being reactive.
With ohmic resistance becoming zero, magnetic leakage also is negated resulting in absolutely no core loss. Therefore, that value which is attained by primary current must also ensure that produced value of flux is sufficient for negating thee.m.f value. Also, change in voltage is in proportion with flux change.
Only when these features match up can you ensure that ideal transformer has a correct theory to function.