One main benefit is that you can change a toon's whole key map set to do something else entirely, with just one command.
Example. I don't know if this is Barney level, but I hope you can follow.
Say your DPS mapped key is 4. Instead of broadcasting the 4 key to all toons, instead it calls a mapped key "DPS Rotation" in a special key map set called "Virtual Combat Key Map".
Then you make more key maps e.g. "Warrior - Protection", "Mage - Frost", "Rogue - Combat Swords", "Priest - Disc" and each one has a mapped key "DPS Rotation" with the steps and actions you want that class/spec to do.
In your warrior toon, under virtual mapped keys you then select that "Virtual Combat Key Map" is now "Warrior - Protection" key map.
In your mage toon, under virtual mapped keys you then select that "Virtual Combat Key Map" is now "Mage - Frost" key map.
In your rogue toon, under virtual mapped keys you then select that "Virtual Combat Key Map" is now "Rogue - Combat Swords" key map.
In your priest toon, under virtual mapped keys you then select that "Virtual Combat Key Map" is now "Priest - Disc" key map.
So when you press 4: your warrior does shield block and sunder; your mage does frostbolt; your rogue does some castsequence with slice and dice; and your priest does wand.
But don't stop at just your DPS key, you can make a whole set: aoe dps, heal relaxed, heal urgent, interrupt, snare, fear, cc, cooldowns, panic button, bandage, drink, buffs, etc, etc.
You might think "that is a lot of work to set up, I might as well just make sure sunder and frostbolt are on the same keys on my toons...." which is fine but not all classes have corresponding abilities, and for some classes you might want to press multiple buttons at once, especially the warrior.
So now what if you are fighting a frost resistant mob, and you want your mage to do something else instead of frostbolt. You can make a second key map "Mage - Arcane" where the "DPS rotation" mapped key does arcane missiles and fire blast. You can make a clickbar that switches the entire virtual key map from "Mage - Frost" to "Mage - Arcane". Or another that gets your warrior into berserker stance to whirlwind and cleave and execute. Or another to make the paladin judge wisdom instead of light. It doesn't have to be a clickbar, you can set a keybind in the mapped keys that does the switch.
The best thing I did when learning ISBoxer was follow this excellent video series by MiRai
https://youtu.be/iqHaH4gbrmo