There are two practical reasons
Surely at some point you may have wondered why while the pizzas are round, all the shops use square boxes to carry them.
There are two main reasons that most people may not know. Initially, square boxes are easier to make than round boxes and require much less space for transport and storage. In stores they reach levels and when the employee needs one of them, he can easily and quickly give it the familiar shape, folding it in predetermined places.
The second reason is definitely more practical. If a pizza came in a round box, it would be difficult for us to get a piece without getting dirty. But since they have another form, using the empty space in the corners of the square boxes, we can easily lift a piece and at the same time not touch or damage the rest.
One could say that a round box would have the same effect if its dimensions were larger than those of the pizza, since there would be enough empty space to provide us with easy access to the pieces.
However, in this case, the pizza during transport would not remain in place and would arrive at our house almost dissolved from the other side. But from the moment it rests on the four sides of the square box, it remains practically motionless so it reaches us just as it left the store.