For an extremely good reason.
Used to soften the impact of what is said next, implying that it is important.
Used other than figuratively or idiomatically: see not, for, nothing.
Source: en.wiktionary.orgIt is not for nothing that Skaldin in one part of his book quotes Adam Smith: we have seen that both his views and the character of his arguments in many respects repeat the theses of that great ideologist of the progressive bourgeoisie. Vladimir Lenin
I grieve for you, how I mourn for you, who are so very dear to me, but again I can rejoice within my heart, not for nothing "have I labored," neither has my exile been "in vain." Saint Patrick
T is not for nothing that we life pursue; It pays our hopes with something still that's new. John Dryden
Not for nothing did Eldridge Cleaver say that the North Korean police made him miss the Oakland police. Brian Reynolds Myers
[R]ace theory is at variance with all Korean traditions; not for nothing did the national language lack a word for race until modern times. Brian Reynolds Myers
It is not for nothing that the devil lays himself down in the ditch. Danish Proverb