The purpose of mastering is to balance the sonic elements of a mix and optimize playback across all systems and media formats. Traditionally, mastering is done using tools like equalization, compression, leveling, limiting and stereo enhancement.
There are a lot of myths and expectations when it comes to mastering. The truth is, that mastering can't turn a poor or mediocre mix into a sonic masterpiece. That's why it is so important to make sure your music has been professionally mixed and is correctly prepared for the mastering process.
A professional mastering engineer needs to have great critical listening skills to identify problems and choose the right tools to solve them. He's able to get an accurate picture of your track across the spectrum and make educated decisions on how it will size up on various sound systems, whether it's a pair of cheap earbuds or huge club speakers. These decisions can only be made with mastering grade D/A converters and high-end monitors, that will not hide any detail.
Hybrid or fully analog mastering with a chain of the finest analog gear can bring in-the-box mixes to life by adding texture and saturation to the sound. Mastering is not the place for dramatic changes, though. Sometimes it's only subtle, but makes a huge difference in the listening experience.
Artists should also see mastering as their last defend line before the release. A mastering engineer listens to the piece of music as a whole and can give you a very important feedback.
As you can see mastering is much more than a technical process, it's a creative job with a high responsibility. That's why it is crucial to put your music in the hands of an experienced mastering engineer. No cheap automated process, like an algorithm can replace the trained and experienced human ear.