Radial symmetry is the property of being symmetrical in all directions. To a degree, Radial Symmetry is important to Modularity.
Round is the most perfect radial symmetry. Hexagons are radially symmetric, and can fit together without spaces. Hexagons form very strong combinations against lateral forces due to fitting together, and are the most efficient form for making use of materials if you make a shape out of multiple hexagons (the honeycomb shape) rather than individual hexagons butted up to each other.
Octagons are radially symmetrical, and can be combined to form a radially symmetric shape, but not without interstices, and they will have symmetrical sections made of multiple octagons, not individually symmetrical.
Polygons with higher numbers of corners tend to be more streamlined the closer to round they become, and the circle encompasses the maximum space with the minimum material requirement for the circumference, but round shapes leave undesirable interstitial spaces and make it more difficult/expensive to engineer solid connection points for modularity.