The board is meant to represent the investors and act as a check on the power of top executives so they don't go rogue and act contrary to the investors interes

The board is meant to represent the investors and act as a check on the power of top executives so they don't go rogue and act contrary to the investors interests. Their job is certainly easier when the CEO is strong and things are going well, but it's still an important role. Often the CFO (and sometimes other C-level officers) will also report directly to the board, or at least have a direct line of communication that the CEO isn't allowed to interfere with.