Find the commit a specific copy of a file is in
Let's say you have an exact copy of a file that you know was checked into git some time ago, but you don't know exactly which revision it's located in. This will tell you which commits added (and removed, if available) that version of the file (its blob). file.c is the name of the file as it's checked into git, and file.c.copy is the exact copy of the file you have.
Export commits that touch certain files
This is useful if you want to copy the entire history for certain files to another repository. Note that if the commits modified any other files, this won't work.
Then you can import the patches into another repository with git am or git apply.