I can’t tell you how helpful knowing music theory and learning to read and write music has been. It is the language of what we do. There are great players who know none of this stuff, but I bet if you asked them, 95% would say they wished they did, and that they had learned it when they were young.
For years I played by ear and it was hard to go back and learn music properly, but I think it was worth it – and I use it every day.
Theory lessons are available from Preliminary up to grade 8 as either part of your regular lesson, or a stand alone course if required. Most Music College courses expect you to have attained grade 5 as an entry requirement.