Before the idea of free electrons which constitute current in metal,it was through that current in conductors flow due to the motion of positive charges.Therefore this convention is still in use.We can understand the concept of the conventional current from the following analogies.

We know that when the end of heated copper wire are at different temperature heat energy flow from the end higher temperature to the end at lower temperature.Similarly when a conductor is connected to the battery,it pushes charge to flow current from higher potential to the lower potential.The flow of current continues as long as there is a potential difference.

Conventional Current Is Defined As:
CURRENT FLOWING FROM POSITIVE TO NEGATIVE TERMINAL OF BATTERY DUE TO FLOW OF POSITIVE CHARGES IS CALLED CONVENTIONAL CURRENT.

Conventional current produces the same effect as the current flowing from negative terminal to the positive terminal due to flow of negative charges.