Why Track Script Changes?

Because writing is rewriting, as they say...

When writing the first draft, most writers just write, ending up with a screenplay sooner or later. But once you have a finished draft and want to make changes (big or small), keeping track of the changes you make can be quite useful: see the changes at a glance, then decide which to keep and which to chuck.

Additionally, if you invite others to edit your script...

It is useful to know what the heck they changed. They tweak a word, add a line, delete some action or remove some dialogue and turn it into a visual such as a smirk or shrug. Some of those changes you might love, others not so much. Being able not only to see what they changed, but then to be able to accept or reject each change can be a critical piece of the rewriting puzzle.

Revision Mode fills both needs

To be clear, revision mode is something you turn on or off. When it is on, all changes get marked in color in the script, so you can easily see what has changed. Because different users show changed text in individual colors, it is also easy to separate your changes from those made by others.