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.
Just like having a good mix of combo cards (e.g. the classic Village + Smithy at a 1:1, or up to 3:2 I hear from some), it’s good to do so with Potions + Herbalists. Branches are nice to play multiple Herbalists, as I’ve been tempted to put Platinum or Gold instead of the Potion (but with multiple Alchemists, of course you’d need to prioritize those instead)