To add to this, some plants will also give off moodlets when your Sims are around them, such as Roses are romantic and I believe Tulips for Energy. If you place a crocus plant or a flower arrangement scented with crocuses near a bee hive, it will keep them calm. You can also use plants to scent other flower arrangements. Bluebells will keep the arrangement fresh forever. Plants can also be used to scent laundry and candles and be used to make Fizzy juice drinks.
EDIT: A couple more things. There's a berry tree in JA that the berries give your Sims different emotions depending on what kind they eat. The plants grown in Granite Falls are needed to make various useful Wellness ointments for that skill and for the Outdoor Explorer aspiration as well.