The purpose of the telephone is to audibly communicate with other people who are far away instantly and efficiently. Before the telephone, long distance communication was done with telegraph machines which were less efficient and took longer to get messages across since only dots and dashes could be communicated.
Telephones are particularly useful for quick communication in emergencies. The key to medical survival in many situations is for the injured person to get medical attention quickly. Before the telephone, it was more difficult for people to get help quickly.
Telephones allow for quick communication during natural disasters as well. That way it is possible for the government to warn people of impending disaster before they arrive. Since electrical signals travel at the speed of light, the government can inform people of coming hurricanes, earthquakes and other disasters that travel considerably slower. When communication was done only through people riding on horses, it was impossible to outrun natural disasters.
Telephones also have the purpose of simple communication in everyday situations such as calling to order items such as pizza or flowers. People may communicate in a normal way to talk about their lives just as they would if they were in the same room.