Why A linear regulator puts a pass element, usually a series transistor, between the raw input and the load and operates it in its linear (partially conducting) region. An error amplifier compares a sample of the output against a stable reference and continuously adjusts how hard the pass element conducts, so the voltage dropped across it changes as the input or load changes and the output stays constant. That dropped voltage times the load current is dissipated as heat, which is why linear regulators are less efficient than switchers.
Watch out The choice about duty cycle describes a switching regulator, where the control element is driven fully on and fully off and the on/off ratio sets the output; saying the pass transistor is eliminated is also backwards, since the series pass element is the heart of a linear regulator.
Linear = valve held partly open (conduction varied); switching = valve slammed on/off (duty cycle).