Have a look at the above graph. It shows the number of downloads for a Windows 10 Mobile app. The app has had no promotion at this point. It was "soft-launched" ready for a big promotional push at the end of January.
There has been one place where this app was mentioned publicly though. On the 8th of January it was included in AppRaisin. Notice what happened on that day--a big spike in downloads.
AppRaisin is a Windows 10 app that helps people discover new Windows 10 apps and games. Anyone can submit an app when it is newly released or updated. This includes the people who have created the app.
Have you built or are building an app for Windows 10? Want more people to hear about it (and hopefully download it)? Then what have you go to lose by submitting it to AppRaisin?