On a flat network, every device can reach every other device. If a camera, printer, or guest laptop is compromised, the attacker can reach the accounting computer too. Segmentation separates devices into zones and allows only the traffic each zone needs.
Segmentation is usually implemented with VLANs and firewall rules. Typical zones in a small business are staff computers, servers, guests, cameras and building systems, and specialized equipment. Farms, shops, and offices with connected equipment benefit especially.