I think the Rawpedia Preferences documentation is pretty self-explanatory.
In my Linux Mint system there’s no difference between using the system wide color profile, or hand picking that same profile from the folder it is located in (as expected). I only have one monitor.
If you want to publish your images on the web, maybe is interesting reading this post about browsers and color management.
On the other hand, for printing purposes it’s better a good custom profile that takes advantage of all the capabilities of your display plus the appropriate printer profile