1/ The hexagonal shape enables a roughly circular, segmented mirror with "high filling factor and six-fold symmetry." High filling factor just means the segments fit together without gaps, which would not be the case if the segments were circular
2/ Symmetry is good because only 3 different optical prescriptions (A, B and C) are needed for 18 segments, 6 of each as seen below 📷 NASA 
3. A roughly circular overall mirror shape focuses the light into the most compact region on the detectors. An oval mirror would give images that are elongated in one direction while a square mirror would send a lot of the light out of the central region #WebbSeesFarther
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