However, if you're arriving at Narita, there are usually some seats left on the city-bound trains, so you can make a "reservation" a few minutes before your train departs, in which case it's really just an assigned seat.

And trains are frequent enough that if the first one happens to be booked, there's always another one in a short time.

The exception is major travel periods, such as Golden Week (April 30-May 5), New Year's, and O-bon season (mid-August).