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Even spreading disease, in a very broad context, can be seen as an evolutionary "good."
There is now research showing that bits and pieces of DNA may be shared between individuals or species through viruses (which are, in effect, reproducible DNA contained in secure packets). The disease may cause suffering for a group of animals or humans, but perhaps in the long term, some of these infections result in beneficial changes over generations.
In the shorter term, the mosquitoes support themselves, the fish, birds, and bats which eat them, and of course, the pathogens and parasites that live within them. Evolution is equal opportunity---all get a chance, whether spiders or swans.
A better question would have been 'where do mosquitoes exist in the natural food chain?'
There is no evolutionary purpose of any organism, but evolution is a series of random molecular events that allow genes to change and organisms to thrive.
Since mosquitoes exist, other organisms have co-developed that make use of them. Bats and birds eat them, for example. Unfortunately some organisms, like bacteria, developed in a way that mosquitoes allow them to thrive, leading to disease in yet other organisms like mammals.
I ask the same about ticks. Why do we have them? They feed on animals throughout their life cycles and pass on all kinds of nasty things to their prey including humans.
They suck the blood from their victims until they are bloated then they fall off and something else eats them getting a nice boost of protein. It seems odd but it is the circle of life.
Mosquitos, like all other life forms, are part of a complex food web. Many fish feed on mosquito larvae, which are aquatic, and plenty of birds and spiders and other insects feed on the adults. Dragonflies and damselflies love mosquitoes. Frogs eat adult mosquitoes, tadpoles eat the larvae. Bats eat mosquitoes. Some eat many mosquitoes, others prefer other diets, but the mosquito does have a place in their food webs.
As for their "purpose", they seem to be an integral part of several ecosystems since they have been on Earth for more than 100 million years, and they have co-evolved with many species along the way. Wiping out a species of mosquito could leave a predator without prey, or a plant without a pollinator. Elimination of mosquitoes might make the biggest ecological difference in the Arctic tundra. If that biomass vanished, the number of migratory birds that nest in the tundra could drop.
In the absence of their larvae, hundreds of species of fish would have to change their diet to survive. And the loss of these or other fish could have major effects up and down the food chain. Many species of spider, salamander, lizard and frog would also lose a primary food source. With many options on the menu, it seems that most insect-eaters would not go hungry in a mosquito-free world.
As for any benefit they provide humans, the answer is none. On the contrary, mosquito borne illnesses (namely malaria) are the number one killer of humanity, period. No other disease or war comes remotely close. Mosquitoes and their pathogens are humanity's worst foe.
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In evolution, it is every gene for itself, "nature" has no larger scheme or design beyond self-propagation
