The flat trigger gives a different feel than the curved trigger, and that is the reason most change to the flat. A secondary benefit to a flat trigger is the reduced trigger pull weight.
The flat faced triggers allow a lower pull weight on the trigger. I know you will say if the trigger shows a 3.5 pound pull, then it is 3.5 pound pull, but pull weights are measured from a near centered pull on the trigger face. With the flat face you can get your finger lower and that lower pull changes the leverage of the pull, which results in a lower pull weight. You can't lower your finger much on the curved face, or the trigger will dig into your finger. When using a pull weight gauge, if the pull weight rod pulls from below the center on the trigger, the pull weight will be lower than a normal centered pull. The lower the pull position, the lower the pull weight. Using a flat trigger you have the option of using a centered pull or you can slide your finger down for a low pull, which will lower the trigger pull weight.
Bob