Since humans populate most geographical areas, they have eliminated many of the predators that lessen the animal population. These are predators like coyotes, wolves, mountain lions, panthers, and even birds of prey. Nature's balance has been adjusted by humans. Now, without the natural prey or hunting the animal population grows way out of hand. This is a disadvantage to humans and to the animals.
Overpopulation causes numerous inconveniences to humans.
One particular example that affects many individuals a day is that of deer getting run over by vehicles. In 1999 NY DOT had more than 60,000 reported incidences of deer being hit. Worse those figures are just for New York. Deer occasionally being hit by vehicles is unavoidable but if the deer population is not as plentiful the costly accidents will happen much less. A further way that keeping the animal population under control helps humans is that it makes it less likely animals will destroy the yards and gardens of people.