That's in there because many SPI devices(especially among those on TinyShields) are MSB first. So, it's simply flipping it for the NRF8001 when necessary, then flipping it back. Common stuff like an SD card, TinyScreen, CC3000 WiFi, etc uses MSB first, and the associated libraries won't work if the bit order is flipped by other code while running.
A better way to take care of this issue is using something along the lines of the newer Arduino SPI 'transaction' interface, but unfortunately very little code out there actually uses this- something we'd like to work on, but rarely get to.