Please Explain SPI Interrupt (SPIE) in SPI control register

Hi there, I have been trying to understand SPI Interrupt i.e. what does it do and why is it used ? Reading about it on the web has not helped me. Can some one please explain it in a simple way ?

It lets the code know when the SPI transfer is done.
As SPI can run at various speeds, it lets the independent SPI hardware shift out bits while other things are going on.