Most often, this is used on broadcast satellites to indicate that radio stations are not multiplexed as subcarriers onto a single video carrier, but instead independently share a transponder.
In an SCPC system, satellite bandwidth is dedicated to a single source.
Another very common application is voice, where a small amount of fixed bandwidth is required.
Some applications use SCPC instead of TDMA, because they require guaranteed, unrestricted bandwidth.
As satellite TDMA technology improves however, the applications for SCPC are becoming more limited.