For ordinary people, owls are simply a rather uncommon species of bird of prey, but for wizards, they are an indispensable part of daily life.
Although the wizarding world possesses a great deal of wondrous magic, they have no concept of telephones. Even the fireplace system (Floo powder) that connects various wizards' homes is merely a means of transportation rather than a channel for circulating information.
Therefore, owls have become their messengers—and essentially their only messengers at that.
Chapter 358