@ekafyi nailed it, yeah. Here’s another very specific use-case just for more context. This one is my real-life.
You’re at work. You’re managing a project. One of the categories is
72 Cabling in a data centre.
For some of the IDs in there you’ve got files that you’ve saved in your file system:
72.01 A thing
72.02 Another thing
Then an issue comes along and it only exists in your email. It’s a proper issue, something you need to track, something you want filed in its own place, as you should. So you check your file system, you see that
72.03 is the next number, you use it in your email, happy days.
… later that week …
It’s hectic at work and you get another thing that you need to save in
72. You check the file system. What’s the next free number?
72.03. Great, go ahead and use it. And now you’ve got two things with the same number, and life will never be the same again. The birdsong stops. Your coffee is always just a bit too cold. They announce the end of NCIS re-runs. Sadness prevails.
You need an index. It’s a pain in the ass, and I wish you didn’t, but experience has shown me again and again that you do.