You'll realize that several control states require the microcontroller to wait for events, e.g. buffer full or empty, packet ended, valid signal received etc. There are several reasons why you won't want to poll the transceiver permanently, if you can't imagine it yet, read the data sheet and application notes thoroughly.
GDO outputs are a perfect way to signal events to the microprocessor. If you don't use the additional GDO lines, you would at least utilize MISO has a dual-purpose signal. But it involves more programming than using separate GDOs.