It is true that there are some fantastic and even very successful musicians that were self-taught music. There are a lot of people that think that this is the best way to learn music because of this. Some people get a certain amount of joy out of knowing that all of the knowledge and skill that they obtained they went and got for themselves. There is nothing wrong with having this attitude, but those people might be missing out on an entire world of possibilities that they didn't know existed before.
People have learned music through a private teacher for years. This is something that has been going since the days of Bach and Mozart. A private teacher is usually someone with either a lot of professional experience or a lot of education when it comes to the specific instrument that they are teaching. There are also private teachers that can teach subjects like music history, music theory, and songwriting.
One of the premier aspects of taking music lessons privately is that the teacher can recognize a student's strengths and weaknesses and teach them accordingly. This is something that almost no person that is trying to be a musician will be able to recognize for themselves. Private teachers are also good and providing a little inspiration or a little push when their student isn't feeling so motivated to practice or get better.
Private music instructors have a ton of experience in music. They obviously love it or they wouldn't be doing that job. This means they have probably discovered a lot of great music in their time that a student wouldn't have found themselves. One of the most important parts of learning to play music is being an active listener. It's akin to how most great writers are also avid readers.
Online Private Lessons
Not everyone has the time to be able to drive and meet with a private teacher on a regular basis. There are many websites that have recognized this trend and have started offering downloadable video lessons for a variety of instruments. There are also a lot of independent private music teachers that offer lessons over skype. This means there is no reason for anyone that is looking to get better at music to use any excuse to not have to take private lessons. The internet has created a world of access for those that want to learn more about the art of music.