Amongst many things, mastering will make your mixes sound louder and clearer; two important characteristics that audiences interpret as sounding ‘better’.
Keep in mind all professionally released recordings have been mastered. If you want to compete for airplay and audience attention, you need to be able to match other records on volume and sound quality. Radio stations, music supervisors, record labels, and audiences are all used to hearing professionally mastered audio, and demand this level of quality.
Often understated is the importance of a fresh set of ears. After months of writing, producing, recording, and mixing, artists and the producers and recording/mix engineers get very close to the music. Mastering engineers offer an experienced, unbiased, third-party perspective, and can catch irregularities overlooked in the mixing stage.
Mastering also ensures that your mixes sound great no matter where they’re being played: be it a professional studio, a car stereo, your laptop speakers, or on your music player headphones.