You may have a file which you want under source control, but you don't want its changes tracked. You can tell git - in this repository only - to ignore such changes.
I have a 'twitter secret keys' file that a build script modifies to insert the keys. I don't want those changes visible on the public repository.
You can tell git to ignore future changes via:
git update-index --assume-unchanged twitter_secrets_file.txt
Note that if you change branches or reset, it will moan if this has been changed anyway. You'll have to git checkout it to revert any changes.