What is the fundamental purpose of marketing?

The fundamental purpose of marketing is to link the productive capacities of the firm with the market for those capacities.

In more detail, marketing is concerned with defining what products the firm's capacities will produce, pricingthose products, communicating the firm's capabilities and value proposition to the market place (promotion), and getting the product from the firm to the customer (physical distribution) ... the classic "4-P's" of marketing.