Answer: The purpose of flashback is to either move the story forward or reveal information about the character. You can use flashbacks for any number of reasons but its primary purpose is to bridge time, place and action to reveal a past emotional event or physical conflict that affects the character.
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The purpose of a Tragic drama is to cause Catharsis, that is pity for the main character who is often a better person than ourselves. The purpose of a Comedy play is to mock people who are worse than us and learn from their mistakes. This was defined by Aristotle, so you can fit this definition into your possible answers.
The typical noun corresponding to the verb "to agree" is agreement.
If you want to refer to the process of a agreeing however, it's "agreeing" , as in "agreeing to this treaty is not easy".