Well, each herb has it's own unique magickal property.
Judging on it's shape, smell, color, and everything that it is, we are able to relate it to something.
Herbs have been studied for a very very VERY long time!
And in that time, each herb has been carefully studied and with that, its properties have been determined for them.
As you know, everything relates to something. We witches call that relation Correspondence. Every magickal thing has a certain correspondence to something, which means herbs have them as well!
SO...when you know what corresponds to the herb you are using, you can decide in what kind of spell or ritual you are going to use it in.
If you are making a money spell, you will want an herb that has a correspondence to money! If you are doing a love spell, you will want to include an herb that has a correspondence to love in your spell.
Same goes for everything else that has to do anything with magick. Correspondences are everywhere...you can find correspondence in metals, crystals, herbs, colors, sounds, symbols, animals, words, numbers, ect ect ect ect ect.....it's endless...everything relates to something =)
So based on correspondence, when you make your spell, you need to watch that the ingredients you are using relate or Correspond to the type of the spell that you are making.
I think i explained it as simple as i could lol...Blessed be!