The way our bodies react to mosquito saliva motivates us to avoid being bitten. Which must have had evolutionary benefits, keeping us away from diseases.
I.e. all those people that didn’t mind them and never got itchy from mosquito bites appear to have died out. And mosquitoes really wish that wasn’t true.
Female mosquitos (male ones don’t bite) inject their saliva in order to get as much blood out as quickly as possible. The saliva contains an anticoagulant to prevent clotting. Most people are allergic to some proteins in mosquito saliva, which triggers the immune system to send in histamines. One of the things those histamines do is to dilate blood vessels, allowing for white blood cells to get to the area more quickly. Dilated blood vessels help the mosquito to get more blood.
So there might be a benefit to mosquitos from their bites causing that itching, because of the dilated blood vessels that come first.
I understand the saliva has a benefit for mosquitoes, but not the swelling and the itching (the “unpleasantness” in the title). In essence, our bodies hung this not-otherwise-useful allergic response on something the mosquitoes couldn’t/wouldn’t/didn’t give up and which was firmly specific to their bites, to single them out.
If there was no saliva our bodies would be pressured by natural selection to pick some other mechanism to make their bites unpleasant. An allergy to their chitin or a phobia to the sound of their wings, etc.
Evolutionary pressure from mosquitoes has probably been no small thing.