If you want to use a Mediation, right now the available ones are:
IronSource - Allows most networks, ANE is made by IronSource, but the bad thing is that it only supports CPI and not CPM.
Appodeal - Allows 99% of networks, ANE is EOLed by the company, my fork was updated 2 years ago. Supports both CPM and CPI. (I had the best experience with appodeal compared to others).
AdMob - Nowadays it allows mediation (fewer options than appodeal), but there is no ANE that supports all the other networks. Distriqt ANE supports admob, facebook and adcolony which is a good start.
You use a mediator because it increases the revenue from ads, however keep in mind that this can only happen if you have a decent userbase (wouldn't work with 500-800 users. I would recommend using a mediator after having 3-5k users).