Personally, I don't necessarily think there is a grand design. I think we, like everything else on the planet, should blend in with as little disruption and destruction as possible, something at which we seem to fail pretty miserably at as a people - we don't act like a native population, we act like foreign invaders.
Interestingly, we don't have much of a purpose like other creatures do. Certain plants will refresh soil, plants also of course recycle CO2 into O2, and serve as food. Animals serve as food and fertiliser - food for other animals and fungi, fertiliser for plants. In truth there is no 'food chain', it's a circle with us, humans, somewhere outside it - nothing explicitely depends on us, if we never existed then it would probably not be an issue. Conservation is usually in place to protect species damaged by us in the first place. Sure, we raise a load of stuff by farming - but it's stuff that isn't supposed to exist in such proportions in nature, and we only grow it to eat it.
Really, I don't feel some humanitarian guilt, I'm just being thoughtful. It's modern man that is the problem - back in the day of hunter gatherers, population was controlled the same as with every other animal - if there wasn't food for us, some of us would die and the population would be at a sustainable level. People lived more nomadically, so the environment had time to recover from each visit.
So, I don't think we really have a purpose other than self-perpetuation. Or maybe just to live. Do we need one though?
For the record, the above is just pondering more than anything else.
Pete