Why A Yagi's performance comes from the mutual coupling among driven element, reflector and directors, so every geometric variable matters. Boom length sets the aperture available, more elements add gain (roughly 1 to 1.5 dB per added director early on) and let you trade gain against pattern, and element spacing along the boom controls the phasing that determines front-to-back ratio, feedpoint impedance and how wide the usable SWR range is. Designers juggle all three, plus element lengths, because optimizing for maximum gain usually costs front-to-back ratio or bandwidth.