Empathy for the listener

The creator of a song imbues certain meaning to it because of their experiences (the subject matter, the creation process, and a million other things). But in the same way that the setting and feeling of a dream is vivid to the dreamer but impossible to convey, that meaning doesn’t necessarily translate to the listener.

This is especially true for solo artists working in the vacuum of their own studio and process, often without the experience of a collaborator or live audience to gauge the effectiveness of their ideas.

Really successful artists often have a natural (or hard won through experience) ability to have a sort of empathy for the listener that lets them get the impact of their ideas across the gap.  

Third parties like mixers or mastering engineers bring an important perspective to these projects, as a sort of ultra experienced test audience, one that may be able to extract meaning and detail from the song in a way that greatly enhances the probabilities of it connecting with an audience.