I think if a group is doing an investigation that they are getting data from various light and dark conditions.. A complete lights out could be helpful. Sometimes stuff only shows up in total darkness. Like some light bouncing into the house from passing car headlights, that sort of thing.
A good example of this is moving or glowing headstones- The phenomena only happens at night because it's car headlights bouncing off the stone that causes the effect.
It happens in houses too- you don't notice headlight light bouncing in your house when the house lights are on, but you notice it when the house lights are off. Or sometimes you do notice it when your houselights are on, and you get that odd corner of the eye ghost sighting. When all the house lights are off, it becomes apparent pretty quickly if it's an exterior moving light causing the sighting or not.
There's pro's and con's to it. I think each method is going to just vary depending what your goal is. Depends on what you are debunking or trying to prove.