Shabbos Sustains the World
Shabbos & Yom Tov
The Ohr HaChaim explains that the world was originally made to last only six days. On the seventh day Hashem blessed it and sent down an added flow of abundance, enough to carry the world through another six days, and so it goes every week. Without Shabbos, creation would return to chaos. This is why Shabbos exists only when Bnei Yisroel keep it. It is also why we may break Shabbos to save a person's life, so that he can keep many more. Someone who keeps Shabbos helps sustain creation, which explains the teaching of Chazal that whoever says Vayechulu becomes a partner with Hashem in creation (Shabbos 119b).
Source: Ohr HaChaim, Bereishis (on Bereishis 2:3)