A serial comma should be used for the two following reasons:
1. It should be used before a coordinating conjunction (before and or or ) with three or more items. For example:
2. In order to avoid ambiguity and confusion. For example:
(In the first sentence, we assume that the writer's parents are Rebecca and Luis, while in the second one we assume that the speaker talked to four different people, to his/her two parents, to Rebecca and to Luis. So, the use of the serial comma is the only way to lead the reader to the correct conclusion.)