Shared light counters the voyeurism of most theatrical performances by setting up an atmosphere of openness and respect. This is especially true when audience members can see one another as well as the performers. We say: yes, you can look wherever you like, and in return we want to know where you are looking and how. We want the right to see if you are with us or examining the lighting grid, because looking at the lighting grid is a truthful reponse to a performance.
It is a scary idea for the performer, because its hard to relax into what worked in rehearsal when you can see that more people are looking at their shoes than at your blood-stained hands. Or that most people are doing that grimacing thing.
Shared light reminds us that we are making the performance for this audience, not for ourselves, or the abstract idea of an audience. It allows them be a gathering of individuals rather than a unified bunch whose reponse we can summarise in the green room with theories that they "liked" or "didn't understand" what we did.
When the audience is lit, a commitment is made that the performance is going to happen in the here and now of play, rather than the there and then of presentation and masterpiece.
If we find this instinctively uncomfortable, maybe its because we're confused about the difference between the theatrical experience and the online experience, where you can be elsewhere from the action and in your underwear, while eating cereal. Come on; be here, with us.