[Q] assume unchanged?

I was looking at the documentation of git-update-index because I thought git-add is the preferred way to do what the command was used for in the old versions of git, and wanted to see if the old command has more features that are missing from git-add.

I noticed that there is --assume-unchanged option, and I read its description three times, but I do not understand it.  What is it good for?  Version control systems are used in order to keep track changes, and if using that option makes my changes ignored, how can it be a good thing?

The manual says "This is sometimes helpful when working with a big project on a filesystem that has very slow lstat(2) system call", but unfortunately it does not answer my question.

Could somebody explain, please?