The best homilist in the world cannot touch all the stories of all the lives gathered with the same energy that people can do for themselves. In interviews, SCC members sometimes say how much more Sunday Eucharist means because of how their gatherings prepared them beforehand. Today in urban and suburban settings, which is where most U.S. Catholics live, proximity does not have the same effectiveness as a natural gatherer. Experiments with non-territorial parishes are one response to this situation. Relevant is something of a hackneyed work, but it weighs in heavily.
Catholics are responsive to experienced relevance.
The reign of God IS relevant, and deserves to be up front and palpable about its relevance to people's experience. SCCs are not the only response to these two factors, the waning of proximity as a gatherer and relevance to living, but they are one. SCCs, however, are not just there to take up the slack. They are a form of ecclesial existence that was the norm in the early centuries. Even the Catechism of the Council of Trent, preoccupied with theology of Church, noted the churchood of House Churches.