These operations are related to generating the output signal, it is usually the last stage in a guitar pedal effect.
So once the input signal has been modified (clipped in this case) it needs to be generated again, so the DACs will produce the new output signal.
2 DACs are used (named DAC0 and DAC1) for this operation (one will be enough, but 2 gives you more resolution). Every time that we want to use the DAC we need to:
Select the DAC (because there is 2 of them, and we need to make sure that we are addressing the right DAC)
Write the desired values on that DAC (the newly generated signal value)
That operations that you highlighted do exactly these 2 things:
dacc_write_conversion_data(DACC_INTERFACE, out_DAC0);//write on DAC
With that line we write the value "out_DAC0" into the DAC0
After that, if we want to use the DAC1 we need to repeat the operation but selecting the DAC1 and writing the desired value for DAC1. In a nutshell, writing into the DACs is not as easy as just giving them a value, we have to select the one we want to use and then write the value.