Do I need mastering?
If you need mastering depends on the following:
If you make music just for the fun of it and not for commercial release than you might not want to master.
But if you are planning to have a career in the music business than you should have your tracks mastered.
A mastered track just sounds warmer, louder and more professional compared to a track that has not been mastered.
It can also be a learning experience for you. Listening to the mastered version of your track and comparing this to the unmastered version reveals the weak pionts of your mixing skills.