What is the primary effect of increasing boom length and adding directors to a Yagi antenna?

A Yagi antenna achieves its directional characteristics through the interaction between the driven element and its parasitic elements (directors and the reflector). Adding more directors and spacing them out along a correspondingly longer boom further focuses the radiated energy into a narrower, tighter beam. Because the total radiated power is concentrated into a smaller area, the forward gain of the antenna increases. This tighter focus simultaneously narrows the beamwidth, which is why distractors claiming beamwidth increases are incorrect.