If I have two content types with a relationship between them in Drupal 8, why would I want to put a reference field in both content types?

If you are user of the example website in the video, I can think of these two use cases:

• I am viewing the Event Detail Page, then I want to know who is sponsoring the event.

• I am viewing the User Group Detail Page, then I want to know what events the group is sponsoring.

Then you would have to discuss the implementation.

In the video, they go for two reference fields, I see it as a shortcut so that you have your Event Detail Page and User Group Detail Page ready. But then you have a new problem, you will have to keep both fields in sync. Having this type of redundancy wouldn't be considered a good practice in database design.

Alternatively, if you don't want to deal with field synchronization, you only need a reference field. Let's say the Event will have the reference field, now we would have to come up with a way to query the events of a group for the User Group Detail Page. A Drupal View would be the most common way to do this.

I think the instructor in the video doesn't want to introduce views at this point of the training, because it is a broad topic, then probably he will go back and revisit the implementation later in the course.

Here's a simple explanation to understand this point:

whenever you need a relation between two content types in both directions without using the views. Let say we have two content types A & B, So if you need a reference of a field from A inside the B and also need a reference of any field from B inside the A then we need to define the reference field in both content types.

If you need a one-directional relationship then you will define a reference field only in one content type but for a two-directional( can also call it reverse relationship ) relationship, you will need to define reference fields in both content types.

I hope this simple explanation will help you to understand the concept.