An Alchemist chain needs Potions to keep going.  Herbalist keeps the Potions on the deck, thus saving the Alchemist from having to bog down their deck with Potions.  In addition, the +1 Buy is a nice bonus; big-draw decks often suffer from the too-much-money syndrome, where they have $20 but only one buy.

Herbalist does have the drawback of being a terminal action, which means that reliance on it forecloses the possibility of finishing with other useful terminal actions (Mint, Remodel, attack cards) without some +Action cards mixed in.