Interestingly, MBTA (in Boston, MA, USA) seem to run a few trains like this. Very early morning departures from Boston, making very few stops, often leaving a couple of hours before the next train.
I don't know the area, but it's presumably to get stock in place for the morning rush hour. And it does make sense, to an extent. If you've got a train with a driver and a guard/TTI as required, then why not run it with passengers on it? You may as well carry passengers and at least attempt to make some revenue out of the service, even
Obviously, it's not suitable for every scenario. You can't really carry passengers into depots or sidings for obvious reasons. But there are some quite lengthy ECS runs, and I don't see why they couldn't carry passengers (provided it's feasible to do so, of course). Could help offset some of the costs associated with running the empty trains, and in this economic climate every little helps!