Absolutely book online. However, you will only get local rates if you book on the Korean site. The English page doesn't offer the same rates. If you wait until you arrive, you will probably have limited or no seat selection. And that means standing in the vestibule.
Anything on Friday afternoon to Busan and coming back on Sunday afternoon to Seoul usually sells out.
That being said, I just checked the Korean KORAIL app for those days. Reservations are available on every train for 1 ticket all day at all times. The local discount is actually only 5% (2,500W roughly).