Not for Nothin'

Not for nothin', a phrase for which Italian-Americans seem to have a particular affinity, probably ascended into the consciousness of more people because of The Sopranos. Like, Tony would say, "Not for nothin', but I'm gonna rip your fuckin' head off." Not verbatim from the show, but you get the idea.
I first heard it years before The Sopranos. A hairdresser I knew prefaced statements with it, and I found it perplexing. "Not for nothin," a double negative, means it's for something. So if you need to preface your statement with it, does that betray your insecurity? If you have to tell me that what you're saying is for something, you're diluting your credibility right off the bat. Maybe you should just keep your fucking mouth shut to begin with.
I guess it's idiomatic, and perhaps it helps to soften whatever is coming next. Sort of like, "Don't take this the wrong way, but...." Because it's always, "Not for nothin', but...." Maybe I should try it out when I'm on the subway: "Not for nothin', but you stink to the point where I'm on the verge of passing out." Would that absolve me of the political incorrectness associated with telling someone directly that they're an olfactory affront? It's better than, "Not for nothin', but I'm gonna rip your fuckin' head off."
Such logic could be applied in any number of situations. To wit:
"Not for nothin', but that was the dumbest fucking thing I've ever seen anyone do."
"Not for nothin', but your ass has gotten really big."
"Not for nothin', officer, but I can't believe who they let carry guns."
"Not for nothin', but that bride ain't beautiful."
"Not for nothin', but that actually happens to be the ugliest fucking baby I've ever seen."
"Not for nothin', but I'm pretty sure your God doesn't exist or is, at the most, second-rate."

You see what I mean? It's a real conversational lubricant. Not for nothin', but try it for yourself. See how it goes.