I think itīs very useful to do that, not only it allows for night adaptation (30 min is ideal but most healthy eyes will adapt a lot sooner)but also it encourages pax to follow the lighted paths and discourages them from starting to grab stuff from their bags and everything.. .They must do that for Take off and landing...also any pax sleeping must be woken up before to and ldg in case of evacuation.. .Hope it helps.