I don't think mosquitoes have any positive thing for this society and they are parasitic and not just humans, even the animals suffer due to mosquitoes. But the existence of mosquitoes can at least make the humans focused on cleanliness, so that mosquitoes can not breed and bring deadly diseases. I personally hate the most to mosquitoes. I feel like it is needless creature to exist in the ecosystem which has no value in the ecosystem.
Although It's hard to tell the positives of mosquitoes and parasitic vermins but still they have some benefits. The species help to maintain the ecosystem. It is used as a food for other species like lizard etc. helpful in flowers pollination. There will be a lot more but unfortunately people are unaware of these advantages.