No,  its not a dumb question. When you are not sure of something, asking about it, is a great question.

 15" rows cover the entire soil surface sooner than 30". If you took a small square box and planted 4 seeds in the middle right next to each other, and another box the same size with 4 seeds planted individually part way in from each corner, if everything else is equal, which box will have leaf surface covering more of the box sooner? Same amount of seeds, but each seed can grow in all directions, vs. only being able to grow out from the interference of the adjoining plant.

 IMO.