You build a great product. Now what? How do you get users?
The next step is marketing. Unless your target market is so tiny you or your sales team can do a direct outreach.
Gaining users and customers is extremely difficult, often unpredictable and potentially expensive. While a bad product/technology/service can set you back there is no guarantee the best will get you success. Users can be unpredictable and the adoption curve for anything new is long.
Sometimes marketing is earlier in the market identification and product building stage so you know the market you are going after, gaps, segments and the value proposition to customers or users. This is product strategy. You can completely botch this step and expend effort on a product/service that is not clearly differentiated and has no market case.
B2C (business to consumer) marketing typically use large budgets to reach a wide potential customer base using traditional advertising channels, and now online and social channels. Even 'lucking' out with hype, viral marketing and rapid adoption requires someone or a team to fuel this hype.
B2B (business to business) marketing is more hands on and about building a predictable pipeline for your sales team to target and close. In some b2b business the sales cycle can be hugely long and expensive and the cost per lead and opportunity measured in thousands of dollars.