CPI traffic gives purchases and activities in the app you can get money for.
Traffic for earning money
Your Android application can get thousands of installs at the market page. You will grow in the category by them.
Promote your app in the category
When you order +1000 keyword installs to reach your top 1 for the super competitive word, you need some 1000 more CPI installs to make this app drowth in rank natural.
Balance promotion by keywords
A new application can not be found at Google Play for its relevant keywords. To make it visible, start with some installs from cheap sources of traffic.
Index your Android app with cheap CPI