When would we use this option/what are the results of using it?
That’s useful to have a field that is a list of something. For instance, on Facebook, one field on the user is going to be “friends”, of type User. But it’s not only one user, it’s a list of users. Another way to see this, on Airbnb, a type of things is an apartment and another is a review, and an apartment is going to have a list of reviews as a field.
Once you’ve built such a field, the actions you can take on it when searching, modifying things, etc. is add/remove an entry from the list.