A social reserve

The sabbatical year, like most sacred institutions, involves centralized, non-crucial spending by the national government. 

As such, the sabbatical year functions like a social reserve, a kind of forced savings of excess and surplus, that could be deployed in the event the society were to under-go genuine environmental pressure like famine, plague or natural calamity. 

These sacral institutions are thus both the products of the social surplus and the embodiments of that surplus. 

The sacral institutions can function properly only if they see themselves as flexible. Once the sacral institutions become too sacred, once the protection of the reserves becomes the objective of the entire social order, then the society loses its ability to adjust to the environmental pressure. With too much sanctity, the society once again becomes subject to collapse.