It's obvious that moving oil in space helps, right? You move the resource from a place it's relatively plentiful to a different place where oil is relatively scarce. Say, from the Persian Gulf to Australia.

The same thing is happening here:

These ships are trying to move oil from a time when oil is relatively plentiful to a time when it's relatively scarce. It's riskier, but if they succeed, everybody wins: they make money, and they do it by lessening that future scarcity.

"Efficient" has a very specific meaning in this case.

https://en.wikipedia.org/wiki/Efficient-market_hypothesis