SPI Wiring Confusion
SPI is a communication protocol, you can take advantage of some hardware optimizations by using the pins where that hardware is available or you can do everything by hand using any pin you like, with the disadvantage of a bigger and slower firmware.
In certain situations, like in the presence of a shield or a library already using those pins, the SPI pins might be unavailable and you have to resort to a software solution (aka bitbang, as you manipulate bits to generate the proper signals), otherwise use of the hardware SPI pins is strongly advised.