Can Git be used instead of Word's 'Track Changes'

Absolutely.

Text changes are just a simplified version of source code :)

Here are a number of really amazing 'non-code' stuff that is happening with Git's content-versioning capabilities:

• Bundes-Git – German Laws on GitHub 
• GitLaw: GitHub for Laws and Legal Documents - a Tourniquet for American Liberty  (with Y's discussion here)
• Using Git for writing thesis [closed] 
• Could Git be used to track bills in Congress? 
• A must watch TED talk about GIT and democracy