As others pointed out, extra comfort on the flat section.

Swapped my factory two piece aero bar and stem combo for a cockpit (Roval Rapide) last year and don't regret it. Aside from looking better, it's stiffer and crispier than my previous setup, especially when accelerating. There are no downsides for me.

If you like to firmly grab your bars near the stem when you're riding (when climbing up, for instance), you may dislike it due to the flattened out shape as compared to a traditional round bar. Bar tape can always be added for extra comfort.