It simply means "open the note I had open last time when I closed Joplin", i.e. how it works currently. While on desktop client it doesn't bother me as much, but on mobile I often just want to make a quick note when I'm out and about and instead of one click open joplin and make a note, I'm four clicks away probably in a wrong notebook and wrong note an it can really break the flow when you have to figure yourself into the correct note.
Now that I think about it, a related feature that might be more useful and serve the same purpose would be if I could create shortcut to a specific link both on desktop and mobile, so I could have my "quick notes" shortcut on my phone's home screen.
Thank you for your question, as this prompted me to think about whan I really want from the feature. For mobile this request seems to exist already.