how important is it to understand sentence construction, advanced vocabulary, and structure to understand literature or adequately interpret a poem.
therory is the basics of the musical language, and i am frequently asked at advanced levels of study to discuss and make decisions in lessons w regard to form, ie unity in longer expanded works, understanding where we are in the piece in sonata allegro form, etc
where harmonic tension lies relative to true tonic, how to enhance the effect of modulations, etc.
you don't have to , but you'll understand the music you play better if you do and you can make better and more informed decisions on interpreting and expressing yourself in those works.
likely not as big a deal in intermediate and early advanced but as you move towards more complex and abstract works and concepts, it becomes more relevant, at least for me and past past few instructors at least
even the theory of orchestration, register, instrumentation etc can help when deciding on voicing in thick textures with 'orchestral' type sound and construction and writing (ie Brahms for example) or in transcriptions, etc. too, it's all related