Is there another purpose?
A few months ago or so, I had this thought that anything worth reading about, as though it would affect our understanding of the music we are playing, should -by definition- be already (evident) within the music itself. And that, if it wasn't, then reading about it wouldn't add anything of value to interpretation and would, in fact, be a form of adding something to it that doesn't belong there in the first place. That's probably true in a priori reasoning and can be used as an excuse to not read up on a composer, which is not my intent at all. I do think though that reading about composers should go further than just they themselves, and ideally we are reading about and understanding something about the world at the time and how a composer fits (or doesn't fit) in with that world around him. And I don't just mean studying visual and other performing arts! I mean, gaining a sense of philosophical beliefs, educational beliefs, religious beliefs, political practices, scientific understandings ... to give you a living -not merely museumish- impression and spark an imagination that is based on some of the lifely impressions the composer himself very well may have been experiencing in his (yes, pretty much his) day. But, then that meets with our own soul ...
But also in understanding a composer's mind frame at the time of writing a piece, and their general philosophy of composing, their music becomes an absolute reflection of their life in ways that are simply not self evident, and in ways that are more powerful. For instance, Chopin's prelude no. 15. We take for granted that the song titled "Raindrop." When you apply that imagery to the song, it's powerful and in some ways moving. Every note becomes a reflection of "rain." But an understanding of Chopin's life, beliefs, and philosophies, shows that the song is not about rain at all. Chopin firmly believed that music shouldn't be tied to imagery, and he never named the prelude "raindrop." It was coined by Hans von Bülow, another romantic composer. Further understanding of Chopin's life shows some evidence that rain could have very well been an influence, but to focus on the song as a reflection of "rain," misses the depth, sadness, and beauty that is the song as a whole. Understanding who Chopin is as a composer, allows us then to understand this song in much greater splendor, which allows us also to interpret the song in greater splendor.