1) You’re too close to the project to be objective. You’ve spent dozens, possibly hundreds of hours working on your recordings. You may have spent an entire week on the hook of just onesong. You’re confident that you know every note, quirk, and imperfection of your mixes inside and out – and that’s the problem.
Your audience probably isn’t going to care about the little waver in the tenor part of your lead-off track, or that the cymbals are hard-panned L-R, instead of 75-75. What your listeners will care about is whether or not your music moves them, even if they don’t know why.
After pouring your heart and soul into the project, it’s common to develop “tunnel vision” and to have a difficult time seeing the forest for the sonic “trees”. You might obsess over something that most won’t even hear, while missing something that others will pick up on during their initial listen-through. If you’ve been working hard on your mixes for days, weeks, or months, you may well have hit a wall; your mixes sound very good, but you can’t make them sound any “better”. You’re not even sure if you need to keep trying.
Every listener has the potential to be affected by a different part of a mix, but there are certain aspects of a recording that have the potential to be more impactful or distracting than others. A professional mastering engineer has the ability to assess which elements of a recording should be highlighted or augmented (e.g. the vocal/solo, bass, drums, or depth/width) and which should be “fixed” or have less focus (e.g. sibilance, undefined low end, low energy, or “oomph”), and has both the tools and the ability to skillfully and musically make these adjustments.
Perhaps most importantly, a professional, independentmastering engineer brings an objective, experienced ear to the project, and won’t be influenced by non-musical factors such as egos, or decisions made during the mixing process which become so ingrained in an artist or mixer that they can lead to sonic “blind spots”.
2) You can’t hear everything that is happening in your mixes. More and more frequently, mixers are creating their music outside of the traditional recording studio. “Bedroom” studios are exploding in popularity as technology becomes more accessible and budgets continue to shrink. This allows for a great deal of creativity, and often yields some terrific results. But it can come at a cost.