The Quick Post feature is helpful in instance where you want to set a reminder.
This is usually just a title that can be used for specific events/newsletters. For example, you can use this if you want to create content for Valentine's Day but are not ready to build a post yet, you could create a quick post as a placeholder and would be able to come back to fine-tune it with content later on.
Please note that adding a Quick Post would not schedule the post. This is done directly in Calendar View.
As you can see, the post shows up on the Calendar, but it is not scheduled to be posted as it currently is only a draft post.
You can go in edit then schedule it if you would like, but it is also a great tool for a calendar reminder!