Mastering requires exactitude, confidence, knowledge of music/audio in general and of various styles and the ability to adapt for each song/project and client. Your CD/EP/Single/Project just isn't ready for release or ready for radio or streaming or duplication until it's been properly mastered. Mastering adds such important items as increased overall volume, clarity and punch, improved stereo image, balanced EQ to help your mixes translate better across various "real-world" speakers and systems.