Why is the rotation system used in volleyball?

Whenever your team wins a point on the other team's serve, your team gets to serve again. Before your team serves, it rotates in a clockwise direction: right back, the server, becomes middle back; right front becomes right back, now the server; middle front becomes right front; left front becomes middle front; left back becomes left front; and middle back becomes left back. Your team won't rotate again until there is another side-out, if your team again wins a point on the other team's serve.

The correct answer is c