Here's why:
* They help form verb tenses: "Has" and "have" are used with other verbs to create tenses like the present perfect ("He has gone") and the past perfect ("She had eaten").
* They stand in for a main verb: In some cases, "has" or "have" can act as the only verb in a sentence, like in "She has a new car" or "They have the book."
Let me know if you'd like more examples!