A change in X11R6.9 requires a change in the startx script I use to launch X.9. I’ve noticed an accumulation of files like.serverauth.NUM (e.g.,.626) in my home directory. By specifying xserverauthfile=$XAUTHORITY we remove the need to create.serverauth files. The change is necessary to enable the change in startx to be made in the X11.9 version of X11.”]
Source: https://taosecurity.blogspot.com/2006/09/eliminating-serverauth-files.html