Hehe, guilty on that. The mechanic is not explained properly, and very few people do actually use the fruits. I know there is a todo, but i haven't decided on when and how to teach this. I'm not a fan of tutorial panels, so i'm looking for something else first (most likely animate the tree, so there is a visual cue).
For the record: If you collect water, one water will let the tree grow. You can use the fruit, which is then visible in the "burning chamber" in the HUD, and slowly used up. During that time, you are faster and can carry more. Similar thing with the repair, only in that instance it uses the third resource (sand).