I've looked through the wiki and the forums. I've heard a lot about RDX in the past, but I still haven't yet gotten a handle on exactly what it offers that other mods don't
Other than the absolutely crazy level of customization available (if you can dream it, you can build it), the biggest benefit was that people who knew what they were doing could fix other users UIs for them. Or design it all for them. It helped a lot of guilds overcome what they saw as weaknesses in standard UIs or as incompatibilities between their raider personalities and those UIs.
For example if you were forming a brand new guild with people who had never played WoW before and wanted those guys to dive right into raiding, you could build a totally custom UI for your healers, and send it to them in-game.
Or for a guild that desired standardization across the board, RDX can be used to eliminate stupid errors like people not having abilities keybound, things like that. Some guilds desired that every healer use the same raid healing setup, etc.