What can we derive from the study of comparing two composers?
In comparing two composers, we learn about the historical development of music as well as different compositional techniques. As Isaac Newton put it, we are standing on the shoulders of giants. No matter what genre of music is being discussed, newer composers respond to the previous generation by bringing in some elements and rejecting others.
For example, in using atonality and dissonance, Schoenberg was rejecting the tonal music system that had been in place since Bach's time. At the same time, one of his great compositions, Pierrot Lunaire, the music has a rich, expressive sound reminiscent of Romantic era composers. Likewise, it is possible to compare two composers within an era. Stravinsky and Webern used atonality differently. Webern's music is based on the 12 tone system, while Stravinsky's music could be called extremely expanded tonality. Composers who want to work with atonality have great examples of different ways to do so, which can inform their own compositional practices.
This type of comparison of composers is helpful in understanding non-classical forms of music. The Beatles created songs that were clearly out of 1950s rock and roll practices, but by the end of their career together, they became experimental in their compositions. In creating bluegrass, Bill Monroe used some Carter Family repertoire, but sped it up considerably and added different instruments.
No one creates music in a vacuum, and we understand music better when we figure out who influenced whom and why one composer goes to great length to differentiate him or herself from forerunners and contemporaries.