Mental connections, I guess. It's useful when Joplin is your knowledge-database, and you want to review it regularly.
In particular, getting shown a random existing note may...
- ...help you memorize your knowledge (like a flash-card system)
- ...motivate you to update this note with new knowledge
- ...motivate you to connect this note to other parts of your knowledge base, by moving it to a more appropriate folder, editing the tags, or linking it in other notes
I've written a simple python script (link) and consider it hugely useful. You probably need some python-experience to get it running.