Russell said that new light should always be consistent with previous teachings -- additional insights, but consistent with what went before, sort of like adding more and more light sources into a room.
If this is what the GB meant by "new light," it would merely mean additional insights, and that would be a legitimate way to keep JWs interested.
However, that's not what "new light" means today.
Nowadays, "new light" is a euphemism the GB uses when they abandon a previous position or teaching and replace it with something inconsistent with what went before. Likewise, "old light" is a GB euphemism for a previous teaching of the organization that even they consider to be wrong.
It's the GB's way of attributing everything they do to God's leadership and turning their errors into virtues for which God should be thanked and praised.
It's their cultish way of making sure that anyone who criticizes them for their errors is seen as an opponent of God and His plans.