Type "buy app install" into any search engine and you will find dozens of services promising thousands of Android installs overnight. What they deliver is emulator traffic: installs with zero retention that Google Play's fraud detection identifies within days. Your install count spikes, your ranking does not move, and your developer account carries the risk.
OnTopRank exists because the underlying need is legitimate - a new Android app cannot rank without installs, and cannot get installs without ranking. The fix is an exchange: real developers install and try your app because you do the same for theirs. Every install is a real person on a real Android device, which is precisely the signal Google Play rewards.