Understanding music theory will help students to develop a truly holistic understanding of their instrument. If you are completing ABRSM grades 6 - 8 practical exams, a minimum of ABRSM grade 5 theory music be completed before starting grade 6.
Studying music theory is like learning a Universal language, it will help you to better understand what you are playing and will enable you to communicate and play with other players and ensembles with more ease and efficiency.
Although it is possible to play by ear and some students prefer this, learning music theory will save the learner time in the long term; to be able to pick up music by yourself and read and understand notation will accelerate your progress and support independent learning.
Head to my theory blog post here to find out more about why theory is important and what resources I recommend.