Read this excerpt from A Short Walk around the Pyramids and through the World of Art by Philip Isaacson.
The traditional art of African nations is a wonderful part of the world's art. Like the art of all people who live in groups called tribes—the people of the Pacific islands, the Native Americans, the Eskimos, the Indians of the Northwest Coast of Canada—it was once called primitive art. But it isn't primitive. It isn't primitive in its shape or in the way it is made or in the deep feelings it expresses.

Isaacson's purpose is
to inform the reader about traditional art; his tone is formal.
to persuade the reader to buy traditional art; his tone is formal.
to entertain the reader with humorous traditional stories; his tone is informal.
to convince the reader to create traditional art; his tone is informal.