Colloquial, inverted way of saying, "for good reason," to emphasize a point. "Not for nothing is Paris called the City of Lights." Used extensively in English literature, including C.S. Lewis, Samuel Butler, and Jack London.
It can also imply consequences, for good or bad. "Not for nothing has Shaq played basketball all those years." Or, in a negative way, "Not for nothing has David Duke earned a reputation for bigotry."
It does not mean "for what it's worth," a common mistake. To see why this does not work, replace the words: "For what it's worth is Paris called the City of Lights."
There is a similar Italian phrase which is not used in the same manner in Italian, "non per niente."
Not for nothing had he been exposed to the pitiless struggles for life in the day of his cubhood, when his mother and he, alone and unaided, held their own and survived in the ferocious environment of the Wild. -Jack London
It is not for nothing you are named Ransom. -C.S. Lewis
Not for nothing is the Urban Dictionary known as the repository of online slang.
Not for nothing do Red Cross volunteers offer aid around the world.