It just makes for less compensations. It is what you want at impact, right?...so if you have it at the top all you have to do is turn to the ball and you have it again at impact.
Personally I like mine a little bent...to avoid hitting hooks/too low.
Its to do with the Geometry of the circle and accumulator no.2
The radius of the circle is formed from your left shoulder (which in motion itself moves in a circle so you still have a center and a radius) - So to keep this radius (a straight line) perfect you must maintain your left flying wedge. So if you take your a dowel (say thanks to Yoda) and grip it so that it goes up your left arm and keep it there allowing the left wrist to cock up and down but keeping a straight line behind your left arm.
Now flat isn't always flat at the top of the backstroke - what if your wrist was turned instead of vertical (yet still flat) at impact fix - The left wedge must stay intact and whatever motion allows perpendicular motion but no horizontal motion (this is the no.2 accumulator out of line condition) maintains the no.1 imperitive - So that flat left wrist would be actually be cupped at the top....