In today's digital world, your prospective customer is going to interact with your website and marketing before they ever touch your product. Therefore, it's vital to bring marketing, user experience (UX), and user interaction (UI) together, especially if you're selling a digital product.
As a marketer with training in UX and UI, I provide prospective customers with a great experience before they start a trial or buy a product. Whether it's copy for an e-mail or desiging a landing page, I write and design for my user.
You can market your product without regard for your user, just as you can eat chips without queso, but who would want to do that?