Some of the paid plugins offer features that others do not. For instance:
AdMob has really good international fill rates. That means that you are more likely to get an ad every time you ask for one.
Chartboost lets you configure how your ads look and give you more control over what ads you get.
There are two plugins: AppLovin and FaceBook Audience network that do not cost you anything up front, but currently 95% of the ads that show are credited to you, the other 5% are credited to Corona. These are our revenue sharing plugins. You still see 100% of your earned revenue in your dashboard and you never have to pay us. It’s just that 1 out of 20 ads that show is on our account.
The remaining options like Appodeal are 100% free to use. You get 100% of the ads and you get 100% of the funds that show up in your dashboard. Appodeal is a mediation system which means they use many different ad providers to assure you always get an ad and they find the highest paying ads for you. You can use Appodeal to get AdMob ads.
Rob