As with any process it depends on your goals. Spending money mastering songs you spent a few hours recording with the intent of producing a demo may not require mastering. See my blog Demo vs. Record.
What is Mastering.
Several things happen during the mastering process. But generally speaking it is preparing of your project for media distribution. For example sending it to a manufacturing plant for CD production. Mastering Engineers can also provide you with several other formats to be used on social media, iTunes, or for the many other online sites or services.
Mastering includes several functions including.
Reviewing the songs in your project and apply processing to insure they are all equal.
Apply corrective EQ to fine tune and enhance the different frequencies and insure an even balance. Apply fades.
Utilize tools such as compression, limiting, to level and enhance different frequencies and glue the mix together. To give clarity to the different parts of the song and insure there are no overs or clipping. Level are also set to insure all song have the same loudness.
Evaluate and enhance (if needed) the stereo panorama. Adjust, if needed, the level between the left and right channel.
Manage the dynamic range of the music.
Insure all songs are spaced properly.
Add meta data.
When the mastering process is complete you will have a project that has been reviewed by a skilled Engineer who will give you project that finished, polished sound.
What improvement can I expect to get with a professionally mastered project.
That depends on several factors. Mastering Engineers can not repair all sins created by the recording or mixing engineer. (garbage in garbage out). But if your project was recorded and mixed by professional engineers who provide clean well balanced work you can expect an increase in quality equal to one or two grade levels. Take a B to an A, or a C to a B, or maybe an A. Mastering engineers may send songs back when changes to the mix will result in a much better end product. It's not unusual for the Mix and Mastering Engineers to begin communication about a project early on.
What's the difference between a Mix and Mastering Engineer. Several things, each is a different discipline, the knowledge and education is different, the tools used are different. Most Engineers have some knowledge of both processes but consider each as a specialist. It takes several years of education and practice to become accomplished at it.
So should I have my project mastered.
If your looking to produce a project that is polished, well balanced, will sound the same on most playback systems, are interested in posting on social media, sell digital copies, making CD's, get television or radio play, then the answer is yes.
A professional Mastering Engineer can assist you in preparing your project for mastering, work with you during the process to ensure the project has the best results. If your Mastering Engineer is close by you will be welcome to attend the mastering session. They should also allow for a review period to review things to insure you are happy with the end results.