The main use would be to see 'more' in the library screen. For example, if I sort on, and collapse on, authors, then I can quickly scan the library for what I am looking for. Same for series. The idea is similar to what one finds in, for example, development environments where I can collapse functions and classes to get a better picture of the organization.
I freely admit that doing so would provide almost the same functionality as the tag browser. I also freely admit that I am not volunteering to build it.
