But what if you’re tracking a thing in your system that isn’t a file at all? What if it’s an email, for example? Or a physical thing?
The ‘your file system is your index’ thing only works if every thing that you track has a file in your file system. (Which could be the case! Or perhaps you could create an empty folder to track stuff recorded elsewhere, but you have to remember to do that.)
Or – more relevant at work – I often have systems where I’m keeping files in separate locations. Depending on what the file is. That’s a pain, but it happens. Then you need an index.