Life form always made its way towards the most adaptable and comfortable environment.
That applies to plants too!
Shapes in circular are more resistant to external damaging. Imagine a square or flat surface tree trunk will provide a suitable angle for animals’ chews and gobbles.
This great wise man on earth inspired the shape of the lamppost. It takes hundreds of, millions of years of self-development to acquire the stage now.
Day in, day out, we take things for granted. So, do we know the moral of the story now…?