Nothing is perfect - and the mismatch causes current to flow.
(if you put your ear to a transformer - you hear a buzz - feel it - and it will be warm (or hot))
and generating noise and heat is wasteful - for something that's supposed to be filling up a battery .... or whatever
Electricity runs from live to nuetral. Your phone charger is a transformer converting 240volts AC to a low DC voltage . Although the phone is not connected, the transformer is still "using" a small amount of current , it,s a bit like boiling your kettle and not using the hot water, after a few minutes it will need to be re-boiled !