This field will appear if you enable the sending of MIDI song number, or if you enable selecting songs by their MIDI song numbers. However, the song number will also be sent if you tap the MIDI icon for a song, so if that is how you want to use the song number, it's true that the field will be hidden unless you enable one of those other options.
The reason the field isn't enabled all the time is that the song edit page is pretty long already and very few people would be using this field. A further reason is that if I enabled this field by default, I should also enable the MIDI program change field to be consistent, but then people might think that field is meant for sending rather than responding to program changes. I would rather do this in conjunction with some kind of reorganization or customization of the song edit page.