I think what they mean is say you have a bit of obsfucated, minified, and or complex JavaScript code you want to reverse engineer. You can start writing a an "unobsfucated" version in JS, pseudocode, whatever you wanted....and use source-mapping to switch between looking/working on that and the original in the debugger all while the browser is actually executing the original code (negating worries that your translation is incomplete or behaves differently, these are concerns during certain steps of the process...obviously you want to end up with a perfect translation in the end)

That would be a useful tool in reverse engineering.