A simple curve on a control surface can do wonders. It can provide tactile feedback (without the user having to direct his gaze to it to differentiate it from others), make the control surface easier to use, and comfortable to touch.
Here are a few examples we see and use everyday.
The EMR and EMR-C have a curved surface, the reason is best illustrated
by the picture below. The profile of our fingers isn't straight but
contoured. The curved surface on the EMR and EMR-C follows the general
contour, and is designed to accommodate a wide variety of finger shapes,
sizes, and gloved hands. It inhibits vertical movement of the finger
while the button is pressed, and increase the surface area without
enlarging the button's footprint.
The horizontal serrations also serve to inhibit vertical movement while the button is pressed, they work better than circular serrations, as you slide your finger up and down the EMR and EMR-C, the increased friction and resistance in the finger's vertical movement is far more pronounced.
The additional surface area also means a bit more weight. The shallow groove, or band, below the curved surface helps reduce the weight gain. EMR and EMR-C weigh in at 0.054 oz and 0.066 oz respectively. Factory button weighs 0.054 oz.
The band also gives the EMR and EMR-C unique aesthetics.
Whereas the EMR has a milspec through hole, is similar in height to the factory button, and has minimal protrusion above the mag release button fence, the EMR-C has a blind hole, continuous serrations, and is taller in height. The EMR-C is easier to access due to its taller profile, it will protrude about 0.08" above the mag release button fence.
Though it wasn't a design objective, the EMR and EMR-C bear a resemblance to the original Armalite AR10's magazine release button. We belatedly realized the similarities, long after the horizontal serrations were decided on. This picture can be found on page 32 in the book The Black Rifle, by R. Blake Stevens and Edward C. Ezell.