As titles go, this one's straightforward. That's fitting for a man like Franklin, who seems like a pretty upstanding, plainspoken sort of guy. Reading his book, we get the sense that Franklin's a straight-talker who shoots from the hip, and seems to tell it like it is. So, the title tells us: what the book is, what it's about, and who wrote it. (Just in case we didn't know.)
It's the life story of Franklin as told by Franklin: this title reminds us that Franklin's both the creator of the story and its subject. With that in mind, we're given Franklin's credentials for writing his own life story: after all, who else could know more about the life he lived than Franklin did?
The origins of the title are a little more complicated, though; Franklin just referred to it as his Memoirs, because the concept of "autobiography" as we know it today wasn't being used back then (source: Atherton and Cawelti). The first portion of the text was originally published in French as Mémoires de la vie privée de Benjamin Franklin, Ècrits par lui-méme, et Adressés a son fils. This translates in part to The Private Life of the Late Benjamin Franklin, which implies stuff that's a little bit different: this title emphasizes that Franklin's dead and, by using a descriptive word like "private," also encourages us to acknowledge our voyeuristic tendencies by getting to see into his secret life.
Finally, humor us a little here, but even the use of "The" at the beginning is significant. This isn't "an" autobiography, or one of a few; it's the authoritative, authorized version of Franklin's life.
Atherton, Eric and John G. Cawelti. "Who named Franklin's autobiography?" ANQ. 8.2 (Spring 1995): p17. Literature Resource Center. Gale. UC Santa Barbara (http://go.galegroup.com/ps/start.do?p=LitRC&u=ucsantabarbara).