Comfort.
For most people, 40-45 degrees is about the angle your hands and wrists want to be at when you hold the handlebars. Straight bars, or bars with maybe 5 degrees of bend, have the outside edges of your hands bent outward. If you hold your arms straight out and downward, like you're holding invisible bars, rotate them between the "flat" position, and the "45 degree" position. You can feel your forearms relaxe as you go back to ~45 degrees.
The selling points of drop bars are 1) multiple hand positions available (flats, on the hoods, down in the drops), and if the bike isn't too big for you, many people find it comfy.
The selling point of truly flat bars is supposed to be leverage in the dirt, which is why they tend to be a mile wide. I've never had a leverage issue where I wished my bars were wider.
....
Thus, bars like his Jones H or my Velo Orange (or many others) that sweep back tend to put your hands -- and the shifting/braking controls, unlike on drop bars -- in a very relaxed all-day position.