I don't know, I'm not a vegetable or animals, but as a human I sometimes consider that their existance in this earth is to compliment human's life. Either to fulfill physical needs or further than that, like some people have dogs as their best-friends.
IMO, when the philolosophers thought about this Q, they made a generalization about what's the purpose of life should be, means that they created a huge idea which can be breakdown to an individual level and thus (hopefully) will explain each person's regardless their situation/condition.
For example, existentialism just stated that life is made of choices. Without making choices and decision, you simply don't exist. This idea is actually can be applied to all humans.