It helps with customs, but it's a big deal for private businesses too. Before containers, you effectively had to have one trusted person's eyes on the cargo at all time -- and still people along the way lightened the load as they handled the stuff. It was almost considered part of the compensation for a longshoreman.

With tamper-evident containers, you have a trusted person watch the container being loaded and locked, and then a trusted person watch as its unloaded at its final destination, and you don't have to care what happens at every point in between.

Makes sense. 99% of what I know about container shipping comes from the movie Contraband and Season 2 of The Wire. ;)