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Acts are containers to hold your story. If you are using Classical Act form, you have 4 containers.
Now add another container or take one away. You story is reshaped, requiring you to make new choices of to tell it.
Often, beginning writers resist Classic Act structure and dump structure entirely, then flounder as they try and find the shape of their story.
Structure is a tool to help you get from the beginning of your story to the end. How many acts you choose to tell it is up to you.