How developer make money by providing apps for free of cost?

In these days, most of the professional buisness are based on apps, games or other applications that are available in diffrent online market's such as Play store, iOS market etc. Even there apps are totally free of cost. Then how they make money for making track in buisness line?

I think most of you know about PPC ads. (If not, read below paragraph otherwise leave it)

In the words of internet PPC means Pay Per Click
. It means you will be paid on the based of clicks made by your coustmers of perticular ads. There are abondent of websites are available on web such as Infolinks, Adsence etc. These sites are giving ads of third party companies for your online webspace. Procedure is just simple- You have to put advertise code into your website and you will get paid 30% (based on there policy) of per click .

Ok ! Move to our main point. When developer make any app or game for publish they simply place there PPC advertise code in there app. Actually, i think this process is more sucessful then selling or placing a amount of your app on store.
The Angry Birds franchise is a great example. On iOS they sell their apps to customers for 69p each. But on Android their apps are free on the Play Store. The reason for this is that in some countries customers can't buy Android apps, they can only download the free ones. Rovio, the devs behind Angry Birds, realised that. They also took into account that if they did sell the apps on the Play Store they would be pirated hugely. So instead on Android they implemented in-game ads. These ads run whenever you play the game. Also with them being there forever, Rovio keep making money every time a person opens an Angry Birds game. This has meant they make over $1 million a month from Android alone just from ads.

My opinion-
In my point of view PPC is the best way to make more for every developer. This method also become a source of your lifetime investment. To illustrate - if someone download your apps for free  in which you already put PPC ads code. Now imagine that he took that app for 2 years in that time period he/she open that app every day it means your app is opened for 365+365=730 times. If your PPC ads company provide you 30 cents for one click i means you make approximately 219$ from one person.

But if you put 50$ of your app then your invesments become 3 times lower. It is also possibility that you loss your half of the coustmers by appling charges on your app.

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