why does energy change when expressed as per mole of reactant?

In chemistry, the amount of a substance is usually expressed in moles as the unit. For example, mole/L is the commonest unit for the concentration of a solution. As the amount of energy change of a reaction depends on the amount(s) of reactant(s) used, the energy change must be expressed as per unit amount of a certain reactant. As the commonest unit for amount of substance is "mole", energy change is usually expressed as "per mole".

Not clear what you are asking. But "per" something always changes the units and the numbers.

Watts is joules per second. 1000 joules in 20 seconds is 50 watts, for example.

1000 joules per 20 moles is 50 J/mol