I thought it was for two reasons: 1) ensure no racer has an equipment advantage over the other, and 2) equipment meet minimum safety requirements for Keirin racing.

I enjoy the Deathare answer: buy NJS stuff if you need cheap gear. Honestly, there's much nicer stuff out there which isn't NJS than stuff which is. One simple example are framesets: Argon 18 makes a nicer track frame than a good number of NJS builders.

FWIW, NJS only matters in Japan Keirin. UCI is cool with lots more variety of stuff - just not the Y-foil.