When we draw a Venn diagram, we use circle to represent a Set. We can use any closed plane figure but most of the time it is a circle. Why? are there any specialty about that?
Circles are easy to draw and conceptually very nice. There is no requirement for anything other than a circle for Venn diagrams with $n \le 3$ classes. Also, the boundaries of two circles intersect each other at not more than two places, which is an important aspect that has connections to some deeper results in mathematics, such as the Jordan Curve Theorem.
However, when you get to $n \ge 4$, you can no longer draw (symmetric) Venn diagrams with circles. $n=4$ requires ellipses, and for prime $n \ge 5$, the shapes get more and more complicated!
If you'd like to read more, this answer covers more details.
Look at this noncircle Venn Diagram: