I found an error in a script and now I have to modify hundreds of php or javascript files. Or some identification data of our organization has changed and I have to change tens of templates. These are situation where you need a tool to find differences and automate operations, as doing it manually would cost a lot of time.
Developers and power users need a tool for identifying differences quickly and easily so that they can focus on more productive tasks. So everybody has a diff tool in his toolkit to find differences between file or folders, merge file or do the same (like modify or delete) on text or data in a large number of files.
My choice is WinMerge, a pretty good freeware. Not best usability I would say, but very good performance, enough for me not to spend money on some professional tools like DiffDog from Altova. And one of best things I like about it is the Windows Explorer Context Menu integration, allowing to compare files without opening the application.
Winmerge also has plugin support, so you can enhance it’s functionality. xdoc Diff is a must imo, it lets you compare Microsoft Office and OpenOffice file formats without having the office suites installed (not the case with the built-in plugins for Word and Excel)