You hook it up to a computer with JMRI, then to a programing track, and use it to program decoders. If you have a Digitrax system, it can be used as an interface between a computer, and the Digitrax command station. A PR4 is quite a bit better.
I have a pr3? Is it worth keeping or not. What is it used for?
If you intend to use JMRI for decoder programming or a computer to control your layout keep it. I have the newer PR4, but I've only just started getting into the whole decoder programming head-scratching puzzle.