Like mosquitoes, flies bite to take blood from the host. Also like mosquitoes, they insert anti-coagulants into the bite to keep the blood from clotting. Horse flies have a particularly painful bite.
Biting flies are not only annoying, they can also be harmful. Some people have allergic reactions to the bites, especially horse flies and black flies. In addition, certain species of flies can transmit diseases. For instance, a type of deer fly in the United States is able to transmit tularemia, which is a type of bacterial disease. Similarly, biting midges can give the blue tongue virus to cows and other animals.