If you are using an editor that supports it, such as vim or emacs, then it makes editing life wonderfully easy. 
For example, when editing a Moodle script in vim, I can put my cursor over a function and hit CTRL-] ... and I'm immediately taken to the function. I often then hit 'y' to yank a copy of the line, then CTRL-T to return to the file I was originally in, then 'p' to paste that line in to that script - this makes it easy to know all the parameter order etc.
It also helps function name completion. I can type part of the function name and then hit TAB to automatically cycle through available completed function names.
To make the tags file be used in vim, just add this to your .vimrc file:
set tags=../../../../../tags,../../../../tags,../../../tags,../../tags,../tags,./tags,tags
For example, when editing a Moodle script in vim, I can put my cursor over a function and hit CTRL-] ... and I'm immediately taken to the function. I often then hit 'y' to yank a copy of the line, then CTRL-T to return to the file I was originally in, then 'p' to paste that line in to that script - this makes it easy to know all the parameter order etc.
It also helps function name completion. I can type part of the function name and then hit TAB to automatically cycle through available completed function names.
To make the tags file be used in vim, just add this to your .vimrc file:
set tags=../../../../../tags,../../../../tags,../../../tags,../../tags,../tags,./tags,tags
That sounds very useful. Thanks. Unfortunately I am not using vim or emacs but perhaps some day I will change to an editor that can use this.