This is for the Arduino boards that have native USB to ensure that the serial port is ready before progressing. The while loop is not needed for Arduino boards where the USB is not native to the processor (such as the Arduino Uno).
Indicates if the specified Serial port is ready.
On the boards with native USB, if (Serial) (or if(SerialUSB) on the Due) indicates whether or not the USB CDC serial connection is open. For all other boards, and the non-USB CDC ports, this will always return true.
Ref: Arduino Reference > communication > serial > if(serial)
