Re: Struggling to see the point?
I think the downvotes for a sensible question seems harsh, but yes it's a counterintuitively more efficient means of harnessing an internal combustion engine's power. I believe an ICE is limited to about 40% efficiency as a theoretical maximum. In a standard car, it's real world efficiency is around 20%, less in many occasions.
As Danny points out above, you can keep the engine set to its most efficient speed regardless of where the drivetrain is. Think of it as an infinite ratio gearbox. In this case, with a diesel, it would be a particular improvement given a diesel delivers all its power at the top end of each gear.
A similar setup (forget which, but was a series hybrid) was reviewed on Top Gear a couple of weeks ago. James May tried to explain it to that blithering idiot Clarkson who as expected cut him off stating it was boring nonsense..