Why does Serial disable PDC?

I am trying to use the PDC for UART communication on the Arduino Due. Why does Serial.begin() disable the PDC channel?

void UARTClass::init(const uint32_t dwBaudRate, const uint32_t modeReg)
{
  // Configure PMC
  pmc_enable_periph_clk( _dwId );

  // Disable PDC channel
  _pUart->UART_PTCR = UART_PTCR_RXTDIS | UART_PTCR_TXTDIS;

  // Reset and disable receiver and transmitter
  _pUart->UART_CR = UART_CR_RSTRX | UART_CR_RSTTX | UART_CR_RXDIS | UART_CR_TXDIS;

  // Configure mode
  _pUart->UART_MR = modeReg;

  // Configure baudrate (asynchronous, no oversampling)
  _pUart->UART_BRGR = (SystemCoreClock / dwBaudRate) >> 4;

  // Configure interrupts
  _pUart->UART_IDR = 0xFFFFFFFF;
  _pUart->UART_IER = UART_IER_RXRDY | UART_IER_OVRE | UART_IER_FRAME;

  // Enable UART interrupt in NVIC
  NVIC_EnableIRQ(_dwIrq);

  // Make sure both ring buffers are initialized back to empty.
  _rx_buffer->_iHead = _rx_buffer->_iTail = 0;
  _tx_buffer->_iHead = _tx_buffer->_iTail = 0;

  // Enable receiver and transmitter
  _pUart->UART_CR = UART_CR_RXEN | UART_CR_TXEN;
}

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