Check out their website, this is what I remember so it may not be completely right: It said upon your first or second entrance (depending on which requirement you meet for applying) you can apply in person at the airport but you must have your own wifi connection and you will still need to do the in-person interview upon entering Japan after you are approved. It does not say it will be reviewed and approved on the spot so I cannot tell you if there is any real benefit for you in doing so. The only advantage that I can see is if you have a complicated reason for applying they may review the documents (which need to be electronic files) to see if what you are submitting is sufficient. There are five categories of applicants, some are not so straightforward when you apply.

For simple cases like having US Global Entry and US citizenship you can just do it online. It's not hard.