Some energy is consumed by simply magnetizing the iron core
Some energy is converted into heat within the iron core
Once a core is magnetized, demagnetization and reverse polarity re-magnetization consumes more energy and takes quite a long time The amount of this loss is proportional to the frequency of polarization reversals.
This happens 120 times per second when operating an AC motor at 60 hertz.
Reducing losses not only saves energy directly but can also reduce cooling load on a facility's air conditioning system.