One for All appeared when All for One gave to his quirkless brother a quirk to stockpile power. Why did All for One gave this quirk to him?
As All Might says way back in Chapter 59 of the manga when he first reveals All For One to Deku, "Whether that was out of kindness, or to bend him to his will, we still don't know".
I initially assumed it would be left open like that, or maybe find a mention later in some arc involving AFO. Now though, since
the vestiges of previous OFA holders, including the first user, are able to interact with Deku
that is another perspective from which we might get new information. So far nothing though.