For me, I see the idea of a square having corners and sides which a circle does not,and a circle having curves which a square cannot, so one cannot be without the other and this sybolises is a sense of dualism.

Also sensing something about the line and the curve being the basic forms for all things in life whether mand made or natural

In the grand scheme of things, it is probably also mathematically related but only Masons have the answer you seek...