Imagine the following scenario:
You have a list of content that is to be sorted by its title on a webpage. But the title may not always be a specific value in the contenttype. It could be a new value made by some logic in the controller, or it could be an optional value with fallback in the view. Often this scenario can be solved by first doing the query, and then later in your controller do the actual sorting of your content. But if you need to fetch content in batches (pagination or similar), that method will produce the wrong result.
In other words, I’d like to be able to sort in my content query based on some kind of virtual or hybrid field. I’d imagine the most common use case would be optional values with fallback, but it could also be on dates or numbers that are to be processed before showing them to the end user.