I'd love to know the story behind that pic!
Back to the point. Fore-and-aft, and side to side restraint is what you're aiming for.
I'd heard it said before somewhere else, and read it again in a similar thread here not long ago, but the idea is that if your trailer shuld break loose on you and roll, your load should be right where you left it on the deck.
If nothing else, at that point, something on your load might provide a handy snatch point for the crew trying to get the trailer back upright. ;8^)