"Everybody tells me that my financial projections will be wrong. With 100% certainty. So why do them?"
You are correct. Your projections will be wrong. For sure. But build them anyway.
First, the actual projections will probably be much less important than what you learn as you assemble them. A good faith planning process forces you to confront questions that you might otherwise neglect. Without a planning process you will, at various times, think about strategy, pricing, hiring, spending or financing. But without pulling them all together into a coherent model, you will not know how decisions in one area will affect outcomes in other areas. You will be rolling with no plan. Having no plan is much much worse than having a plan that will be wrong.
Second, if you hope to get outside capital to fund your business, you need to explain your business to outside investors. Yes, people get excited about ideas and they may even "get it" when you tell them what a great business this will be. But before they put actual cash into your company, they will need to believe that financial success appears plausible (or even likely) based on a reasonable set of assumptions. They also need to see that you understand your business and the levers you need to pull to generate a success.
Finally, "winging it" by yourself or with one or two co-founders may work fine, but as your team grows you really do need a plan to guide everyday activities and your decisions. And when your projections prove to be inaccurate, just adjust them and, if necessary, adjust your plans.