Of the major functions of business—product development, customer service, accounting, operations, and marketing—the one that should always be given top priority in an entrepreneurial venture is marketing.
The other functions are important, but without marketing you will not have sales and without sales you will not have cash flow and without cash flow you will not be able to pay for all the other functions (except by going into debt, which is simply borrowing against the cash flow of the future). Put differently:
Without sales, it is very hard to sustain an ongoing business.
Consider this to be Rule Number One of Entrepreneurship.
Before your business makes its first sale, it is nothing more than a set of unproven ideas that you are spending money on. Some of those ideas may be good and feasible. Others may be bad and/or impractical. You can't know which are which until you test them by selling the product.
And that's why, when I consult with entrepreneurs who are still in the planning stages, I try to get them to streamline their start-up process by eliminating most of the other things they are inclined to do—such as leasing office space and buying furniture—and focusing on effecting that first, real sales transaction.