Why A PIN diode has a wide undoped intrinsic (I) layer between the P and N regions. At RF the stored charge in that layer does not have time to recombine each cycle, so the diode behaves as a linear resistor rather than a rectifier, and the value of that resistance is set by how much DC current is flowing through it. More forward current means more stored carriers, lower resistance, and less attenuation, so a simple DC control current gives a smooth variable RF attenuator or switch.
Watch out The choice about a large reverse voltage describes how a varactor is operated, where reverse bias sets junction capacitance; reverse biasing a PIN diode just turns it off rather than giving variable attenuation.
PIN = current-controlled resistor. Vary the DC current, vary the RF loss. Varactors use reverse voltage; PINs use forward current.