Narrow bandwidth signals require extreemely stable reference and stability. The frequency accuracy is critical for this application.
Narrow SCPC audio and data services typically use either costly internal or external reference sources to lock the carrier.
This reference modification isn't necessary for the majority of signals that TV and radio satellite hobbyists equipment is capable of receiving. Most consumer gear is limited to Symbol Rate above 1k. In recent years, some consumer receivers and tuners were capable of tuning SRs below 1k if provided a frequency stable signal. The C1/C2 PLL LNBFs provided the stability to tune several radio services such as the low SR NPR mux.
If a hobbyist has capable hardware, this modification will allow reliable reception of very low SR (bandwidth) signals.