I think you're expecting to use the uart to do the bit shifting for you. The code fragment you show simply shifts the bits out one by one. The value to send is in the Accumulator, the RRC shifts the accumulator right by one bit into the carry bit, the carry bit is copied to the port bit. We then toggle the clock bit to clock the bit into the 6276 and then we do this for the 8 bits. We then do this for the 6 bytes then toggle the latch pin.

This is called 'bit bashing'!

I normally keep the uart free for serial comms - it helps with debugging.