Charging users to download your apps is a simple, convenient monetization model. After creating your Merchant Account, you set prices for your apps in the Developer Console. You can optionally include advertising or use In-app Billing to sell additional features or content.
This model could work well for any app or game, but might be particularly relevant to those with extensive features or that address a narrow niche in the market. Certain categories of apps, such as games for children, should be monetized by paying for them up front instead of advertising or in-app purchases.