When customer details, prices, or financial figures live in several places, they drift apart, and every meeting starts with arguments about which number is right. Designating a single source of truth for each kind of data, and syncing other systems from it, ends those arguments.
It does not require one system for everything. It requires clear ownership of each record type and integrations that respect it. For example, the accounting system may own invoices, the member database may own contact details, and the planning tool reads from both without editing either.