Basically, a pitch is a presentation of a business idea to potential investors. Broadly speaking, people pitch a business because they need resources. It could be done to raise startup cash, or if an organization needs a partner or resource to meet their goal. Other businesses pitch to potential customers to sell their product.
Well its also necessary to know that a pitch to an investor may not be always just for startup capital. For e.g., a business may need an investment when it has an unexpected but very profitable order beyond its current capacity. Such a pitch is easier to make because investors feel far more secure with the purchase order in hand.