When the longshoremen shut down the ports in Los Angeles, ships full of goods were either left standing at the docks or anchored off shore, as the giant cranes needed to off-load them were silent. In turn, no goods could be loaded for export, in effect crippling the nation's economy.
The Port of Los Angeles is a major shipping gateway to the rest of the United States. The advent of container shipping made it easier and faster to load and unload cargo, but it also made the system more vulnerable. Giant cranes lift the containers out of the holds and settle them on a waiting flatbed truck. The truck either sets off cross country to deliver the goods or heads to a railway yard. Sometimes the containers go directly from ship to train.
When the Los Angeles ports shut down, no one was there to operate the cranes. Ships already docked couldn't leave, costing the shipping companies time and money. If the goods were perishable, the delay was even more expensive. Meanwhile, clients waiting for the goods were out of luck.
A long work stoppage also causes delays in getting shipping schedules back to normal, similar to delayed flights at an airport causing airport congestion. The difference is that airlines can often pull another plane into service, but a ship sitting at anchor filled with containers simply has to wait.