Think about it this way – once Roon is displaying your genre tags, they’re all going to show up in the genre tree somewhere, and you can organize them as you see fit.
The only reason to use the mapping feature is for genres you don’t want to show up in Roon – either you want them hidden completely, or you want them mapped to a genre that already exists (meaning, everything with the file tag Memphis gets displayed as part of Memphis Soul, or also the aforementioned Post-Bop example).
The genre browser is probably where you want to focus your energy in most cases – think of the mapping piece of this as more a gatekeeper, helping you configure which tags genres are actually imported into Roon.