It's a quick way to communicate your position on the issue.
If you're a 2 and your partner is a 9, then it's not worth arguing about. Your core needs are not being threatened. Accept their position and move on.
If you're a 9 and your partner is a 9, then it's worth exploring further. Seek to understand each other's position by asking open-ended questions.
Use the compromise ovals to come up with a temporary solution if you get stuck.