The Zealot mastery Path of Alchemy is designed to enhance the Zealot's direct healing and direct damage abilities. Unlike some of the other healing archetypes in Warhammer, all three of the Zealot's trees enhance all of their aspects, including damage, healing, buffing, and debuffing. The differences have to do with playstyle.
The Path of Alchemy, overall, enhances and adds single target and direct abilities for both damage and healing. Single-target is the opposite of AoE (area of effect) and direct is the opposite HoT/DoT (over time abilities). The benefit of these types of spells, and therefore this Mastery Path, is that the Zealot does not have to worry about reapplying over time abilities, and is very effective in situations where there is only one target, be it a tank that needs healing or a boss mob that the group is fighting.