Stan Lee insisted it was to show Martin Goodman, the publisher, that Stan and Jack sell comics, not inflated headings on covers, so Stan made up a ridiculous heading so long it had trouble fitting, but the book sold like hot cakes.
I think it must have been more planned than that, because you couldn't have got Nick Fury Director of SHIELD without a reputable character everybody knows. That's why I think Fury was introduced as a war character, met Reed Richards in issue #3, and Cap in issue #13. I think Stan and Jack needed someone with a war background the super heroes would know and trust, to have as leader of SHIELD. Stan Lee says he got the idea for SHIELD from Man From UNCLE the TV series, but I think it was the other way around, because when Uncle came on TV it reminded me of the comicbook series SHIELD. So I think Lee and Kirby had SHIELD planned years ahead and fit Nick Fury to the position.
The way Stan Lee puts it, Fury was just done on a bet, but how could you create Fury, and then think what to do with him later? Fury and SHIELD must have been something floating around in their heads the whole time and they just needed to flesh him out in a war book. Yes, the Sgt Fury book looks like it replaced the Incredible Hulk book, but there has not been a more influential character like Fury in Marvel comics. This can't be just a happy accident of circumstance?