The benefit that the index gets you in such a situation is that sometimes you are working on two unrelated changes at the same time, but they both touch the same file. Git (using the -p flag to the add command) lets you interactively select portions of a file to add to the index.

The interface is pretty simple: it just shows you each small piece of diff to the files you are adding, and you say whether you want to include that bit of diff in the index or leave it in the working directory. Alternately, it can drop you into a view of the diff in the editor, and you can add or modify diff lines as you please.