Most people interested in music who want to learn instruments understand that basic techniques are important. Because of this they will practice basics like scales, chords, etc. However, most people do not spend any time learning music theory. Music theory is often overlooked by many starting off musicians, and this actually can make it take longer to get a solid grasp on their instrument. Music theory is important because it is the building blocks on which almost all songs are made, the exception being certain modern electronic art pieces.
It is important to have a good understanding of music theory because, just like being able to read music, a musician needs to have a basic understanding of what is going on in the song. For example, certain groups of notes are meant to be played together in a phrase that gives a moment for a rest before the next phrase, but if a musician does not recognize the two phrase they might play them into each other and end up creating a rushed feeling, or for horns and winds players, they might run out of breath.
Music theory is more than just complicated rules that were used by composers hundreds of years ago. It is the basic foundation of almost all western music. Almost all pop songs on the radio today are made with music theory using the basic principles that make music popular, such as repetitive and recognizable chord progressions. Music theory is not always complicated, and in most beginner and popular music songs the music theory behind the music is actually quite easy to understand.
With this in mind music theory comes to life and becomes more than rules of music, but instead turns into the basic strings that holds music together. This makes music theory an essential part of a musicians knowledge to not just be able to read the notes, but understand exactly what they are playing. The best ways for a musician to gain music theory knowledge are to first either research and read up on basic music theory or hire a teacher/tutor to teach them the basics of music theory. It is then helpful to do a quick analysis of the songs that they are learning to see real life examples of music theory in practice. This will both help them understand what music theory does, and also give them a deeper understanding of the songs that they are playing. This deeper understanding will give the musician an added edge over other musicians as they will not simply be robotically playing notes, but will be bring the music to life and able to add their own feelings and style to the piece and perform at a higher level.