One advantage is that you can map a button to a folder, allowing you to drill down into a whole different set of items. This isn’t functionally different from what you can achieve with a numpad + modifiers, or even just a regular keyboard, but it’s clearly nicer and requires less memorisation.
It’s also not just “custom icons”, it’s a live display — e.g. in the top image of the linked post, you can see a display on one of the buttons showing (presumably) an upcoming calendar event.
It’s also not just “custom icons”, it’s a live display — e.g. in the top image of the linked post, you can see a display on one of the buttons showing (presumably) an upcoming calendar event.