A mediation platform connects your app to many demand sources through one SDK and decides which ad to serve for each opportunity. Today most mediation runs primarily on in-app bidding, where networks bid in real time, with some waterfall instances remaining for sources that do not bid.
Choosing and configuring mediation is less about the dashboard eCPM and more about delivery: which setup produces the most reliable value from the impressions your game can actually serve, once bid rate, win rate, show rate, latency and failures are counted.
Every added network SDK also adds weight, start-up time and crash surface, so more demand sources is not automatically better.