Sprint Reviews are held at the end of the Sprints for a few very specific and very important reasons, which if missed or perverted, would turn the entire event into a waste of every ones' time and would lead to misaligned work that would be doomed to fail in the market.
Left unchecked, they can drift away from a great window of opportunity for the entire team into an expanding black hole, sucking everyone's energy and morale into oblivion