The serial comma — the comma before and in a series, as in the title of this post — aids in understanding. It clarifies. It makes reading more effortless for the reader.
The Chicago Manual of Style supports the serial comma’s use. (The comma is sometimes referred to as the Oxford comma.) The AP Stylebook does not, in most situations.
Roy Peter Clark, the distinguished writing teacher at the Poynter Institute, prefers the serial comma. In an essay here http://www.poynter.org/how-tos/journalism-education/256443/ap-style-should-adopt-the-oxford-comma/, Clark goes so far as to say the AP Stylebook should adopt the comma.
This clever infographic from dailyinfographic.com outlines the issue.