It's fairly standard practice now. On a daytime flight, it keeps the bright sun out. If your aircraft is for example doing circuits while waiting for a landing slot, the way the sunlight appears to swing around the cabin can upset nervous fliers. And as pointed out it makes the video screens harder to see.
On a night flight I'd guess it just sort of contributes to a calmer cabin environment with fewer reflections and possible distractions.
In any case, I find that given the height that long haul aircraft generally cruise at these days, together with what might be haze or air pollution, it's rarely possible to see very much out of the windows other than at take-off and landings - when all the blinds must be up.