Disclaimer: I am not a tech, and I don't have a lot of brake experience. However, I have installed my own pads, so that qualifies me as a I-club technical forum expert.
I thought that the metal clip was to ensure that the pressure of the piston was evenly distributed along the brake pad. If you noticed, where the brake pistons contact the pad, it is really just a circle (e.g., a hollow cylinder). So to distribute the pressure on the pad, a metal plate is added.
But I am just speculating . . .