This analysis was written by FlashingFire.

At first glance, Ward is a rather underwhelming spell. A temporary resistance buff certainly has its uses, but it's not in nearly as much demand as healing or movement spells. It does, however, provide significant benefit to the caster.

See, Ward is a spell with extremely few prerequisites to cast. There is no need for an ally to be injured or debuffed, or for an enemy to be within range. The only requirement is to have a nearby ally. What this means is that Ward is an easy source of regular experience and Faith experience. These things apply for Warp and Rescue as well, but those have fewer uses and generally need to be conserved.

Any unit with the Ward spell appreciates the extra levels it can provide, especially on Maddening. You can cast it while standing next to Byleth for maximum efficiency. But Manuela, Constance and Anna are especially appreciative of Ward, as it lets them more quickly reach higher-tier movement spells (Warp, Rescue, and Rescue again, respectively).

In short, while Ward may not often be needed for its stated purpose, it is a convenient bit of training for units that don't have any other job to do for a turn.