Simple said - we need prototypes as they represent the preliminary step before the minimum viable product (MVP), which is part of every Lean Startup approach.
For more detail, you can check the following two sources. First, what is a MVP?
"A minimum viable product (MVP) is a concept from Lean Startup that stresses the impact of learning in new product development. Eric Ries, defined an MVP as that version of a new product which allows a team to collect the maximum amount of validated learning about customers with the least effort. This validated learning comes in the form of whether your customers will actually purchase your product.
A key premise behind the idea of MVP is that you produce an actual product (which may be no more than a landing page, or a service with an appearance of automation, but which is fully manual behind the scenes) that you can offer to customers and observe their actual behavior with the product or service. Seeing what people actually do with respect to a product is much more reliable than asking people what they would do."
And second, what is the difference between a prototype and a minimum viable product (MVP)? According to Peter Baskerville
"While both product development approaches are designed to validate hypothesises (guesses), as a rule, prototypes will be developed before a minimum viable product (MVP). This is because a business should first establish its proof of concept via a business case/model before releasing a product solution into the market. A prototype is usually a better way to test the proof of concept and market need because it is quick to produce, flexible to adjust and is cheap to produce.