Why not waterfall?
As time goes on, product requirements will change. Building in an Agile way from an MVP allows you to respond quickly to change.
In the old days of Waterfall development, you would build out a comprehensive list of requirements using the information you had at that point in time.
Then the development team would go away for 6, 12, maybe 18 months.
During that time, the world has changed, and the product you requested is no longer what you need. You have wasted a lot of time and a lot of money, and lost customers.
When you build a minimum viable product you can reduce costs while building customer relationships. You can get quick, valuable feedback and evolve your product based on these learnings. You will end up with a product that delivers value quickly, earning you money to reinvest in further development, whilst refining your product.