I think it effects growth and how happy they are. The gardener is the one who reaps the rewards of your plants being happy. Feed them, water them, keep the pests away, and you get reagents and pet food. In your spell book, click the " ? " on the tabs and you'll get more info about what you can do for them.
Putting things near a plant that it likes makes it grow faster so that you can obtain those bountiful harvests that much sooner. To get the best results, the likes should be in place BEFORE you plant the seed, not after.