The Zen Garden is sort of a game too. It makes you money. You take care of the plants (water, feed, etc.) and they pop out coins for you. Unlike in the other modes the coins do not disappear in 7 seconds if you do not claim them - but do disappear after 1 minute or when you leave the Zen garden screen.
The chocolate has several functions: It speeds up the snail (if you have purchased him) and keeps him awake for an hour to collect coins for you. The chocolate also speeds up full grown plants for 5 minutes. Plants do not ask for chocolate, but do enjoy it.
If you have a full basic Zen garden you cannot get more plants, however. You can buy a night garden for plants that sleep in the day and a water garden for water plants, but I'd recommend waiting until you are pretty flush with money before getting those. The cost vs. return will be worth it in the long term; It takes a long time to make your money back from those other two gardens.
Hope this helps!
