The Devout is one of the endgame jobs of Final Fantasy III. It is behind the Earth Crystal, alongside the Summoner and the Magus. It can be considered the equivalant of the White Wizard job from the original Final Fantasy.
The Devout is the "advanced" version of the White Mage job: It can use Tier-8 spells and their previous tiers have far more charges. Statwise, the only area in which White Mages are superior is, strangely, Strength.
In the original Final Fantasy III, Devout comes at a somewhat awkward time. The job is located in the Ancients' Maze, right before the Crystal Tower, and therefore Eureka. Thus, you'll likely only have the Devout for the span of two dungeons before swapping it out for the Sage. You don't need to switch, but why wouldn't you?
Due to the Sage being nerfed to be on more equal footing, the remake's rendition of the Devout gets more use. However, while it is a straightforward upgrade to the White Mage, it still starts at Job Level 1, meaning that leveling the job level will be necessary to get the full benefit. Thus, how hard it outclasses the white mage is somewhat up for debate. Strangely, the Devout was given a bell curve-esque growth for its MP, meaning that spell slots 5 and 6 will outpace the earlier four tiers or the later two.
In addition to getting more charges for the tiers 5 and 6, Devouts get access to the exclusive Tier 8 of magic: Tornado, Holy and Arise. Arise functions as a full-revive while Tornado and Holy are magic attacks. Holy in particular is considered the peak of White Magic in opposition to the Magus' Flare. Though, keep in mind that spell charges are limited, especially in the remake with a whopping 8 casts...
Devouts are extremely flexible in terms of party composition due to their status as a healer. The Elder Staff allows the user to cast Cura on a single character free of charge, making it a must have - especially since the Devout has less casts of it than a White Mage. The Rune Staff is recommended as a second staff due to it increasing Mind (if the Holy Wand is unavailable). As strong as Holy is, it eats into the Devout's 8 casts of the highest tier, which also includes Arise which can resurrect a party member with full HP