Risers bring your hands upwards, can be 'pitched' more. Since I lever the balance of the bike weight to lift the front tire, higher hands work.
I would think it can increase the cockpit space as if the hands are higher, so can the saddle.
The angle on risers varies, I've 2 kinds - both are not horizontal after the bend.
The upwards angle bar is to be run pitch foreward, a leverage bar I used for jumping\climbing, the ones that slope back and down I use for xc and for hand comfort.
Better leverage means less hand comfort. The angles are not as extreme as in my crappy drawing.
I put flats on Sat, back to risers Tues...man I love my ODI hex off grips.
Bar swap, re-setup maybe under 10 minutes.
My reasons for risers are comfort and leverage.
Flats I run for weight (my risers are both heavy duty dh) and to adopt a more correct xc body foreward\areo position for speed.
I'm not racing so comfort and leverage won out.
As far as the set back of the ends to the center....IMO that may have to do with elbow and wrist position.
And if you 'ride' compared to cycle.
Since I do mtb and pull and lift the bike a lot, I prefer little setback as my elbows will be out and my wrists are angled by the setback -it is prefered to have the bars closer to straight if doing any forceful stress on the wrists.
A riding style that is more subdued can have the sweep back more as the elbows come in and a more areo position adopted. If you pull the front wheel up with this type of bar, you have to stick your elbows out and pull up with your wrists bent.
Sorry about the simple illustrations - you probably get my points.