We're not scientists (or Emma), but we know that mixing in the flour in stages allows the fat to coat the flour, keeping gluten from developing. (Gluten equals dense and chewy instead of light and fluffy.)
Two more reasons you might add flour in increments:
1. You have to mix longer to get everything incorporated, and more mixing means more gluten.
2. Dumping it in all at once and then turning on the mixer makes flour fly everywhere.
How we do it differently:
1. We find that we don't have to mix very long at all for our batter to come together. It's really no more than doing it in stages—and less if you count the times we inadvertently leave the mixer running in between additions.
2. We use our trick of draping a towel over the bowl, which works like a charm. Dump in your flour by lowering your bowl or even taking it off the stand to make it easier. Drape a kitchen towel over the top, making sure it isn't hanging in the bowl, and hold it secure at the back of the bowl (but keep your fingers free so that the beater can move). Then turn on the mixer on low and let it run for a few seconds. Turn it off, peek to see if the flour cloud has died down, and mix more if necessary, just until everything is combined. Here's a picture of the towel:
The photo above is from this post, a review of the Beater Blade:
• Beater Blade for KitchenAid Stand Mixers
(Image: Faith Durand)