An MMP sits between your app and your ad partners. Its SDK records installs and in-app events, matches them to the ad interactions that preceded them, and reports results per network, campaign and — where passed — creative. It also sends postbacks so ad platforms can optimise.
The MMP is where the most useful UA diagnostics happen: joining attribution with downstream retention and revenue by source. Granularity depends on what each network shares; some self-attributing platforms expose far less creative-level detail than others.