I'm thinking this morning about the value of performing a piece, rather than just learning it well. For me there is an additional level of learning that takes place when I prepare for performing a piece. Of course, I'm thinking about performance right now in the context of this week's spring Ensemble concert, but I'm casting my mind back over the various works I've studied and comparing the ones I've learned (albeit very well) and those I've learned and then worked on them to perform them (either in public or for my teacher). There's a difference and I think it helps me to understand why performing is so valuable to a student.
When you're not just playing for yourself, you pay much more attention to timing and intonation. Someone else is going to notice that note that's a little sharp or flat and rhythm that's not quite right. So you do a lot more careful listening to the notes that you're playing and maybe you practice with a metronome to be sure that the timing is correct. Not that you don't do this normally, but you do it with more attention in preparation for a performance.
I guess I should pay close attention to dynamics in my regular work, but often I neglect it in deference to just being able to play the piece. But performing for others, the piece has to "sing" and have sections that are soft and lovely and sections that are more strident and loud. I want the listener to like the piece and be drawn into it.
When I'm learning a piece, most often I don't focus on knowing it from memory. But for performance, even if I don't learn the whole piece from memory, being able to play the important phrases from memory frees me up to focus on what I'm doing, frees me from the notes on the page. If I work on the piece long enough, some parts just naturally seem to get memorized, but dividing the piece into phrases and being able to practice each phrase without looking at the music seems to enhance the sound. That said, even though most of the piece is in my memory (both muscle memory and intellectual memory), I play better when I have the music in front of me. I may only refer to it it intermittently, but removing the stress of having to rely only on my memory for the piece results in playing that is more relaxed and natural. Plus, if you are playing with others, all of you may find the score a relaxing prop. Knowing the other parts is really important in listening for entrances, etc. But having the music there is a great stress reducer (and I find emotional stress and nervousness do not help a performance).
Performing makes you take the music to another level. More preparation enhances that level. I know I'm ready to perform a piece when I can just sit down and play it without worrying about notes or measures that might be problems. Not that I always get to that point!