On my large screen, I find that having two windows is often quite useful. Sometimes three, but not for long.

Main use cases are:

  * Look at a "template" file as I'm working.  
  * The "occur" window 
  * Really, any grep output
  * Compilation window (This is how I run all tests)

Good list. A few more for me:

    * Implementation/header pairs in C++
    * Language REPLs (e.g., Python)
    * Quick calculations in Calc while coding
    * Checking dates on the calendar
    * Magit status
    * Diff buffers (especially diff-buffer-with-file)
    * Listing neighboring files with dired
    * Notes on a code base/task in an Org mode file