The problem with standard MP4 ads is that they cannot be streamed

This results in buffering; while the ad is buffering, the user may switch apps, resulting in a wasted ad pod/impression

In contrast, the playback of a streamingready ad (such as HLS) starts as soon as the player accesses the stream

There’s no disturbance in the UX, which is a win-win: user is happy, ad is served