1) it is needed because the parent forum is being swamped with threads on a specific topic
2) Having a forum for a particular topic can attract traffic. Both those people visiting the forum and from search engines.
We've ended up with hundreds at AVForums
It means that someone wanting to find discussions on a particular subject might have real difficulty finding the appropriate forum. For example, where would you post a question about Android TV boxes?
The answer is
Home Audio Video Forums -> Player & Recorder Forums -> Streamers & Network Media Players
We've found a website with way more forums than us who have created an elegant solution. Trip Advisor, which is an absolutely huge forum obviously doing something very right.
Here is Trip Advisor's Florida Forum, Travel Discussion for Florida – TripAdvisor
You'll note that it displays the threads from this forum and all the subforums (places/major attractions in Florida) rather like (but better presented than) this addon we commissioned the Forum Digest by Waindigo
I think Trip Advisor's thread list is better presented because the subforum is in a separate (first) column which provides essential context for the thread title (second column). The number of thread views is omitted and I don't think that's a great loss.
Note that above the list is a helpful drop-down list of all the subforums as opposed to the verbose list of subforums like we have in Xenforo. If there are a lot of subforums, this list can be long. E.g. TVs
Replacing that verbose list of subforums with a simple drop-down list also means that the breadcrumb would be immediately above the list of threads for easy backtracking if the viewer decides they have landed in the wrong place.
Also when all the threads are displayed from the current forum and its subforums, it means there is more visibility of the latest threads from all those forums. One of the biggest complaints from users when forums are split up is that they have to visit different forums to see the latest threads. Creating this 'digest' of threads overcomes that problem.
I would like to suggest that Xenforo core to have an option globally or on a per-node basis to display a 'digest' of all the threads from the current node and its children. Also an option to display the simple subforums drop-down list like Trip Advisor does rather than the verbose list of subforums.