The "Other Bookmarks" makes sense when you understand that it is where you can put bookmarks that you don't want to see on the Bookmarks Bar. Let me explain.
The Bookmarks Bar is like an accordion. If you only have a few items it can be short, with everything visible on screen, or it can extend offscreen into the ">>" overflow zone. You can drag and drop items anywhere along the accordion. If your visible area is full and you insert another item into the visible area, the last visible item will slide into the ">>". Likewise, if you move or delete something from the visible area, the next ">>" item(s) can slide back into view (depending on the character length of the item labels).
Now, imagine you have an item towards the end of your accordion called "My Secrets", which is offscreen in the ">>" zone. And then imagine you remove some items from the visible area, and without you noticing, "My Secrets" has slid on screen. That is a reason for the "Other Bookmarks" folder, i.e. a place where you can put bookmarks that you don't want to see on the Bookmarks Bar.