mosqitos are food for bats. Its all part of the ecosystem. Fish eat mosqitos larvae. Frogs eat them.
Whether you believe in God or not mosqitios and all living things serve a purpose they are all part of the ecosystem.
Birds eat ticks. Parasites are also beneficial to the ecosytem.
I think everything in life does have a purpose. You might have to think about it and look for it but its there.