I was scrolling trough the ATmega328P datasheet when I came across a rather fascinating statement about the way in which the data part of the SPI interface is buffered. I can't seem to understand why the double input buffer is nescessary. The following statement can be found on page 217:

The system is single buffered in the transmit direction and double buffered in the receive direction. This means that bytes to be transmitted cannot be written to the SPI Data Register before the entire shift cycle is completed. When receiving data, however, a received character must be read from the SPI Data Register before the next character has been completely shifted in. Otherwise, the first byte is lost.

Can someone please briefly explain to me why the double input buffer is nescessary. Thanks in advance.