The Daddy long legs flying bug is possibly a de-evoloution of a species of fly. I guess their purpose is to be a part of the food chain, they get eaten by birds, spiders, frogs & toads and other small creatures. I think you need to realize that EVERYTHING in the world exists for a purpose hence we call it the ecosystem. Their existence helps keep the food chain going.

Oh and as for humans helping keep the balance, no we actually interfere and disrupt the balance, nature works wonderfully well without us on the grounds of "In the times of plenty more predators are born, In the times of lack less predators are born as the food becomes scarcer." This balance between predator and prey worked well up to the industrial revouloution where we really started messing about. A lot of people will contest this but don't you see it's true for humans too we work in the same way, when population is low and resources are plenty we will breed and peace rules, when population is high and resources are scarce we will starve, get diseased and since we have no natural predator other than man itself hunt ourselves back in to line. Funny isn't it we give that hunt a name, oh yeah War.

Everything has a reason it's just up to us to discover it.