For me, that purpose is gardening and ecosystem creation. Working with nature to create abundance for both the human and for nature.
I had kids, got busy with work, wanted out of the suburbs, got some land in the country, and started planting stuff. Years later I have what I call a food forest. I have replaced lawns with fruit trees, bushes, vines, flowers, all woven together to mimic a natural forest - except everything in it is edible.
It has changed my life completely.
I just think there's something about being outside in the sun, feet on soil, hands in dirt, planting shit. Watching it grow in the background of your life. Going out and checking on it and watching how much bigger it is getting. Watching life find it, and now you get to live WITH the natural world, not isolated from it.
Birds making nests in your trees, frogs making home in your ponds, seeing bees all around your flowers, butterflies everywhere. Rabbits and squirrels moving in, then owls moving in to eat those. It's this wonderful trophic cascade that you started, and it's incredibly rewarding.
Plus, being able to just walk outside and get chemical-free nutrient-rich dinner is amazing. You can see my food forest in my post history if anyone is interested in seeing what I'm doing.
I actually think a big reason depression is such a big thing these days is because we have all become so disconnected from the life that we evolved in for the last 6 million years. We evolved walking around savannahs and forests, pulling food off bushes and trees, out in the sun, feet in the soil, hands in the dirt. We couldn't be further from that, living inside our isolated homes, cars and offices. It's no wonder we're all so depressed.
I think gardening is the secret to happiness.