Combination of all those, especially bump steer when running high upper arm angles.

Angled servo also cranks in more Ackerman late in the travel, unless you set up for all the throw, in which case the inside angle actually relaxes.

I've run both through the years and have no real preference either way, generally set the car up for whichever method the manual seems to prefer. These days it's flat whenever possible, likely for the CG over any other concern.