What is the purpose of a descriptive essay?
to portray a subject using vivid language
to give details about a personal experience
to express some thoughts or ideas to readers
to convince readers to accept a specific point of view
to provide information about a topic to readers.

Descriptive essay, from its root word or base word, describe, it portrays a subject using vivid language or adjectives. It describes an event, a story, a person, it may describe the whole context with imagery. It also actually gives vivid details about a personal experience. However, to convince a reader is a persuasive text. And, to provide information is informative essays. 

The details that best show evidence of tribal traditions are, "never in the history of his people had a man married a woman who spoke a different tongue” and "it has never been heard," was the verdict of an old man speaking a few weeks later.