When a file changes on disk outside of my text editor it's nearly always because I:

• modified it in another editor, or
• used git to switch branches.

In both of these cases I would like my editor to notice the change and start showing the new version unless I have unsaved changes. I can make emacs always do this by putting:

   (global-auto-revert-mode 1)

in my ~/.emacs. I've not found a situation where this makes emacs do something other than what I would want it to do.