This might be best explained through an example.
Imagine if you had a property that contained the embodied carbon of an element (kgCO²/kg).
Then you had an expression-based property that converted the kgCO²/kg to an RGB value.
Not sure exactly what that would look like, perhaps R = kgCO²/kg x 50, G = kgCO²/kg x 10, B = kgCO²/kg x 75.
Concat Expression R & G & B.
The end result would be a graphic override that could on the fly change the colour of an element based on its Embodied carbon.
At the moment you need as many GO rules as there is values.
Endless other applications
Hope that makes sense.