florence,
"Not for nothing" is an idiom. It doesn't have to be grammatically correct.Technically it is a double negative and wouldm't t be used in formal writing.
As an idiom "Not for nothing" has two different meanings.
1) the double negative
"Not for nothing" = for something
It's not for nothing that I am saying this. There IS a reason I am saying this.
2) New York slang
"Not for nothing" is a translation of the Italian idiom "non per niente". In Italian double negatives are permitted. You could translate it as "I'm not saying this for any personal reason" or " I have nothing to gain by saying this."
It is used in the same sense as "just saying".
Well, Jimmy, but you look like a gay in that suit, just saying.
Well, not for nothing, Jimmy, but you look like a gay in that suit.