Why use SPI?

I2C lets you connect many devices to a single bus (two wires, plus ground) so it would be the preferred choice if you needed to interrogate a substantial number of devices, perhaps fairly infrequently. However the speed of SPI could be more relevant for situations where you need to output rapidly (eg. a LED strip) or input rapidly (eg. an ADC converter). - Nick Gammon

SPI debugging

The code below demonstrates how you can achieve this by sending debugging “prints” out from the device you are testing, via SPI, to a second processor. The second processor simply sits there awaiting incoming bytes, and then dumps them out its serial port. Thus you effectively get a second serial port, via SPI.

This is suitable for debugging inside an interrupt service routine (ISR) because doing the SPI.transfer does not use interrupts.