There are various Black hat techniques and buying reviews is one of them. 

Reviews are very important to both the Apple app store and Google Play. 

For the app stores to know if your app is real and legit, they monitor the number of downloads. 

However, since you can easily buy downloads, they look at your reviews which is harder to manipulate. The more quality reviews you have, the higher you rank. Buying reviews can make your app look alive and relevant.

To avoid black hat techniques such as this, brainstorm on how to get a steady stream of organic reviews from your users. 

Though it is notoriously difficult to get users to review apps – even those they love – resorting to the black hat technique of buying reviews should be discouraged since it might harm your ASO efforts in the long run.