I get that it's a compiler backend for the GPU of the Raspi, but what does that mean? I mean, can you compile arbitrary C code to run on the GPU? Is this for making a GPU firmware? Divers? games? Bitcoin miners?

Any and all of the above.

Although the VideoCore in the Pi is particularly interesting because it's responsible for the early stages of the Pi's boot process (the ARM cores are actually turned off when you initially apply power). Right now, that's all a big Broadcom-proprietary binary chunk; good compilers would be the first step towards freeing that code.

Whatever you want. It runs on each GPU core. You can do a lot of DSP for example.