They are basically passive enhancements to certain groups of Druid abilities. They are also used to power bird and bear forms instead of mana.

You are never hurt by having them so generally you want some and you can go after certain kinds if you know you are playing a role.

Going to be doing a lot of healing? Get some curative herbs. Damage? Grab offense. So on and so forth.

Utility is what you get when you have a partial fail, higher herbal forage skill % will get them less often. The moon phase is kind of like a hidden herb.

Clear as mud?