Answer: Well the historian would know who wrote the and when it was written. Let’s say example your reasearching about 9/11 and all of a sudden you found King Henry the 3rd’s diary, how’s that gonna help you. Sourcing would help them determine if it is related to the subject their reasearching about and they would know that it was reliable due to the person who wrote it, like lets say a peasants Who never knew kind Henry wrote that ”King Henry the 3rd was Santa” and than a priest who was close to Henry wrote “hes a cruel man with the love of meat”, the historians know they belong to the same time line but their obviously gonna go with the priest‘s writing since its “source” is more reliable.