There are three opportunities that you have during a general election campaign where you can substantially move the needle of public opinion. One, is your convention speech; two, are the base; three, is the selection of your vice president.
Convention speeches are powerful tools to bend the curve of public opinion. George H. W. Bush's 1988 convention speech is a great example. His son's speech was also quite powerful.
The initial attraction of a political convention was that often the outcome was not preordained. There was at least some element of surprise. But, now it's like tuning in to a movie where you already know the plot and the ending. It's just not that interesting.