Differ lets you compare the contents of two directories simply and visually. This is especially helpful in using Xcode; you will often need to compare your current project with the previous version when testing or narrowing down the cause of a bug. The difference between two revisions can be created by command line, but this is not a useful way to compare projects. Differ simply presents comparison result of two directories by drag-and-dropping. It allows you to compare projects without a version control system. And you can open files in FileMerge by double-clicking.