Naofumi doesn’t just hold up a shield. He has to catch and shift the impact of opponents’ blows to stop them, and parry them as well.
Naofumi’s self-made defense style allows him to break opponents’ weapons, thus aiding his party. Though this doesn’t work on Legendary Weapons. He can do Shirahatori, and grab weapons and arrows out of midair. He can interfere with magic as well, and support magic is his duty.