It's a sensor that informs its users of it how much control is needed for a networked server that can experience volatility in the form of network traffic spike

It's a sensor that informs its users of it how much control is needed for a networked server that can experience volatility in the form of network traffic spikes. The numerical calculus from PID controllers helps provide that crucial information. A bang-bang controller would, for example, provide very sub-optimal control, as volatility doesn't work under simple on-and-off models.