If I can drive a custom property based on a reference dimension in the Custom properties, why is Configuration Specific Properties necessary?

If I set a custom property ("Length") equal to a reference dimension in the part ("D1@Sketch10@XPart1.SLDPRT"), and the value updates each time the configuration changes, why is there a need for Config Specific props?

I cannot think of a place where the properties would need to be separated into "Custom" and "Config Specific."

Am I missing the logic behind this?