In a traditional substation, if you want to add a new protection relay, you run a new copper wire from the CT terminal block, through conduits, across cable trenches, to the new panel. That's days of work, physical space, and risk.
Nothing is free. IEC 61869-9 introduces challenges that analog purists love to point out: iec 61869-9
: While building on the concepts of IEC 61850-9-2 (the "Process Bus"), IEC 61869-9 provides more rigid requirements for instrument transformer manufacturers to ensure interoperability between different brands. In a traditional substation, if you want to
: Designed for high-bandwidth applications, such as high-order harmonic analysis. Research and Informative Papers In a traditional substation
: Used for quality metering and time-critical DC applications.