When you build a Herbalist, it shows a circle with an influence area. What does this area actually mean? The people served by this building or the area where it gathers herbs? In the second event, how far away from the town can/should it be to be effective and beneficial?
The circle denotes the area the Herbalist will gather herbs from. In order to maximize the effectiveness of the Herbalist you should have it in a forest with a high density of fully grown trees, as shown in this video by quill18. In other words, the more trees the better. This applies to gatherers and hunters lodge as well. So in order to get the most out of your herbalists, gatherers, and hunters lodge the best thing to do is to pair them up with foresters, who can create large forests. Just make sure you don't allow them to cut down too many trees in one area or it'll reduce the size of your forest.