Linked from
The 11 pages that link to Agricultural surplus, each with the reason it gives.
CivilizationRelated: Surplus can support specialists who do not farm full-time.
Yayoi periodRelated: Rice surpluses could support larger settlements and specialized work.
Urban revolutionRelated: Stored surplus could support people working outside food production.
GranaryRelated: Granaries make surplus durable and usable beyond the harvest season.
Pre-industrial societyRelated: Surplus could support specialists, towns, rulers, and trade.