trying to figure out why new map provided by salesforce if old map already exist. every one providing difference is there but i am unable to find exact answer,
newMap A map of IDs to the new versions of the sObject records. Note that this map is only available in before update, after insert, and after update triggers.
oldMap A map of IDs to the old versions of the sObject records. Note that this map is only available in update and delete triggers.