Other than adding flowers to the plant, what is the purpose of plant food?. I know that radioactive plant food can heal you plant fully, but what about the other 2 plant foods? they just seem like they waste money. I have used plant food on a plant beofre and it actually made the plant MUCH harder to sell!

The effects of plant food aren't fully realized until at least the 2nd generation after you use it. If you keep putting it on your plants, after a few generations, you'll have a lot more flowers and the plants will bring higher prices.