Well, imagine you create a 1-person startup. You make the perfect tool, but you have no customers. How do you acquire them? You can try pumping your own personal network, organically market yourself every chance you get (like HackerNews comments or Facebook posts). However you can, with a relatively low investment, reach an order of magnitude more users with digital advertising. Your startup makes a great product, right? So you'll naturally acquire at least some of the users who see your ads.