What are data available and data required in SSI/SPI
You are mixing two completely different things. Writing data to the register makes it shift out, so there is data available for transmission. The data is shifted out by using the clock signal.
The devices must operate with the clock so that they have a chance to see stable data, i.e. if the master update the data on falling edge of the clock, the slave must load the data in on the rising edge of the clock.
The slave needs to have the data stable for some time before the clock edge, and the data must be stable some time after the clock edge, or the slave won't be able to capture it properly.
The master must then of course make sure that the data is available for long enough time before the clock edge to fulfill the requirement of the slave how much before clock edge it must be stable.