There are other reasons to learn written music, even if you're popular musician. For example, you may want to become a studio musician, so you need to learn to play quickly (in a few minutes) a complex piece of music to perform immediately. Studio musicians need to do this all the time, and reading from a score is the easiest way to achieve it. You may need to write music for other musicians (for example, wind instrument players and pianists). Finally, reading music will help you to learn theory and have a better understanding of music.