It's used for reading the 64 bit counter.
I was using the memory mapped register method for a system timer until I realised that just using the coprocessor interface for it was far simpler.
That single line I provided reads the 64 bit counter via the coprocessor interface instead of the memory mapped interface. That's all. It grabs the whole 64 bits at once and puts them into two registers. My example uses a1 and a2, or if you like R0 and R1. Same thing. I believe they cn be any general purpose registers though.