Moving a file (within the same file system) is an atomic operation on most file systems, but writing data is not.
If you don't do this and you're overwriting a file directly and the write fails for some reason, the data from the old file will be gone and you'll only have a partially-written new file in its place.
This also helps with systems that continuously poll files and watch for changes. If you have, say, a compiler watching your file, you don't want it to start compiling a partially-written version of your file and give you some strange error just because it happened to poll before the write finished.
