Cost per Install, also referred to as Pay per Install, is a widely used pricing model in online marketing.
The advertiser only has to pay when a user installs his application. Since CPI campaigns intend to increase the number of downloads for an application it is widely used by marketers to increase App Store and Play Store rankings. However since the ranking algorithms of Google and Apple have changed, it is not that easy anymore to improve rankings just based on paid installs. Metrics like engagement, social buzz and others are now also taken into account by Google and Apple for calculating the app ranking.
Nevertheless CPI is widely used by marketers as the standard pricing model to drive user acquisition campaigns for apps in the App Store and Play Store. Combining CPI campaigns with CPA campaigns the ranking of the app in the App Store and Play Store can still be improved and hence will result in organic downloads.
You can find a detailed overview of all pricing models available in this article.