Jinesh,
This line
while(TRXEM_PORT_IN & TRXEM_SPI_MISO_PIN);
Make the MCU wait for the CC1120 to indicate that it is "ready" to accept commands.
The next one
st( while(!(UCB0IFG & UCRXIFG)); )
makes the MCU wait until the last bit of the last byte has been transferred over the SPI serial interface, this helps avoid situations where you overwrite the SPI port with new data before the old data has been sent.
Regards,
/TA