OM SRI SAI RAM
Deepak ji, Sai Ram!
The answer to your question is very much available in the "Introduction" to SSc by Sant HemadPantji which comes before the First Chapter of SSC.
Infact, the chapter "Introduction" in SSC deals only with the necessity of SSC when Gurucharitra is available for Parayanam. This was discussed in detail by Hemadpant.
Baba Bless U always.