In normal law if you put continuous pressure on the sidestick you'll get a constantly increasing pitch attitude. This doesn't work well for landing, as you'd need to apply pressure, release to neutral, then fine tune.
By introducing a nose-down command in the flare, you can apply back-pressure and hold it, as you would when flaring a traditional aircraft. Airbus didn't word it well in the FCOM. In direct law the landing is almost identical provided you're stabilised in pitch before beginning the flare.