Purpose/Value of Transcribing
This is pretty simple… you don’t know how to play something and you take the time to learn it and write it down as you go. There’s nothing wrong with learning solos by memory, but the added value of writing them down is that you have something that can be edited over time and is a reference when you want to revisit the song (it’s common to work on a song for a while and then move on to other material, which makes it a high probability that you’ll forget most of what you’ve memorized if you don’t play the song regularly). You’re also likely to spend more time to get the details correct, which is what spurs growth, especially when trying to figure out foreign sounds. The most important purpose of doing transcriptions is to train your ear. As you develop the ear to figure out solos quickly, the faster you’ll be able to translate a sound in your mind to a location on your instrument in performance. So, transcribing can be thought of as having three primary purposes…
1) Decipher in full detail what someone else has played for study (both technique and vocabulary)
2) A document that you can reference later, either for re-memorization or to edit for greater accuracy (you’re fooling yourself if you think you’ll get it all the first time)
3) Train yourself to quickly figure out parts played by other instruments on the fly in performance (such as a hook, head or part another musician wants you to play) or to be able to realize on your instrument the cool parts your mind is feeding you.