Let’s say a North Carolina shipper can fill a truck with many small shipments headed for several western states. An FTL carrier will move the full load to a breakbulk facility in a central location close to the end points. The breakbulk facility will sort out LTL loads for transport to the final destinations.
Savings occurs because the freight travels fewer overall miles with the FTL carrier, using only the fuel required for that truck to the breakbulk facility. If you use several LTL carriers, they travel more total miles and may all be traveling the exact same route before turning off to final destinations. This multiplies fuel cost.