[JohnMakin]

I think it is to separate command line input from output -

$ echo "Hello!"

Hello!

The $ denotes that this is a terminal command, and anything that is not preceded by $ will usually be some kind of output. That's certainly how/why I was using it.

[galbar]

On top of that, I find it useful to know if a command should be run as a normal user ($) and root/administrator (#).