Food production
Herbalists are often used to help with food production in an early fortress as they do not require farm plots or other setup. While plant gathering is less efficient than farming and significantly less reliable, it provides a relatively immediate payoff in comparison. Plant gathering is also an excellent way to acquire above ground plants in order to bootstrap above ground farming (the only alternative is to wait for a human or elven caravan and hope they brought seeds).
Above ground, this skill works best in areas with thick vegetation like forests; is less useful in sparsely vegetated biomes like deserts or badlands; and is completely useless in mountain biomes which cannot have shrubs. Underground, plant gathering can be a high risk activity in the caverns, and production can be easily outstripped by farms. However, once a non-dry cavern has been opened, underground shrubs will begin to appear on all underground soil or muddy tiles. Herbalists can continue to gather from these to supplement your farming production, at no risk.
The number of plants in a stack a herbalist gathers depends on his skill the same as with quality levels in earlier versions - for every 'bump' in quality, there is one more plant, starting with 0-1 and capped at 5.
When a herbalist picks some plants, he will carry them to the food stockpile himself, even if his food hauling labour is disabled.
 Gathering seeds
Plants gathered by herbalists can be eaten or processed to provide seeds which can then be planted in farms. For this reason, it is popular (if not necessary) to gather above ground plants early in a fort's existence if one wants to grow above ground crops.
 Speed of training
Dwarves gain experience in plant gathering every time they pick over a shrub whether they are successful at harvesting anything or not. Training a completely unskilled dwarf to legendary status in this skill requires the harvesting of 600 plants, and can be done in less than a couple years. A legendary herbalist not suffering from any skill penalties will never fail to harvest a stack of four or five plants.