The Knight will cover allies in critical health status. So, if you're fighting enemies that rely solely on physical attacks, you can give him dual-shields and use Defend to absorb any and all damage for your teammates if you keep them at critical health.

In addition, only the Knight can equip Excalibur, though you won't get that until the final dungeon.

Just experiment with what jobs you like best. All the jobs in this version are reasonably well-balanced, so any party with a dedicated healer should succeed.