As your practical skills improve, it is important you develop a solid understanding of the building blocks of music, whether you are a singer or a playing musician. Knowing how the pieces fit together in music you learn will make you a better and more confident performer.
For those wishing to take advanced practical exams (ABRSM Grade 6 and above), or to apply for a Bachelor Degree program at university, ABRSM Grade 5 Music Theory (or equivalent) is a requirement. Therefore, unless you have been learning theory alongside your practical music lessons (I incorporate the two), you may find yourself needing a music theory crash course.
I strongly recommend you buy this book! The AB Guide to Music Theory: Part I is something every musician should have in their toolkit.