They're not, they're centrists. At least that's what they would be called in the UK.
They're essential to maintaining the status quo because the overton window points at them and declares that there is left wing opposition to the right, when in reality that is as far left as capitalism and the establishment are comfortable with, and it's really the centre. Chomsky talked about this, saying there is a narrow window of acceptable thought, but by maintaining vigorous discussion within that window the public are led to believe there is a broader political spectrum than there actually is.
It should also be noted that when push comes to shove, centrists always align with the right wing over the true left.