On the web, it's obvious that nobody will find your site by accidentally typing its URL. On mobile, many people still seem to assume that users can find apps on stores without any marketing to drive them there. That hasn't been true for at least five years.
Dominating mobile game companies like King (Candy Crush) and Supercell (Clash of Clans) spend over 500 million USD yearly on marketing. Unless you have that kind of money, you can't pay your way to the top of app store listings, so your marketing should probably focus on some very specific niche.