I'm not a radar controller, but consider this: If you don't read it back, and just press the ident button, there will be two, three or more other pilots, wondering if you heard it and waiting for your readback so they can then transmit in turn. Only the radar controller will know you have responded to the instruction. A needless few seconds of uncertainty that can be avoided by reading back 'squawk ident' then pressing the button.
And it's a mandatory readback, as mentioned above.