I am simply not understanding the benefit of this feature. From a YouTube tutorial I learned about AF-ON button on few cameras. In my camera (Nikon D5600), it is located under:
Autofocus Settings -> Assign AE-L/AF-L button -> AF ON
When I enabled this, the camera stopped focusing on shutter-button half-pressed. It completely now relied on the AE-L/AF-L button for me to click to set focus. In the field I tried to capture some photos. On a sudden action, I forgot to press this button and therefore camera was not able to focus. When I enabled AF-ON from the menu, my camera was in Manual Mode with AF-C and 9-Points Focus Area.
I restored the button to its default settings of AE-L and AF-L. When does it makes sense to use AF-ON ?