Linux

I believe macOS's default partition is case-insensitive but not case-preserving. I remember having to check the HUnit (unit testing library for Haskell) in a special partition because darcs barfed on a file whose case changed.

I remember that the BeFS in BeOS was also case-preserving but not case-sensitive. Scot Hacker, the author of the BeOS Bible, relayed an explanation that resonated with him. (Bear in mind that this was pre-2000 and the computing landscape was much different. This was also the time that macOS was born in.)

The short of it is that it's for usability. The average person doesn't really differentiate between upper- and lowercase; at most, it's just aesthetics. If they want to find their resume, they don't care if it's spelled resume, Resume, RESUME, or even rEsUmE. Why should the computer require that they conform to a design decision that was made decades prior?

Since then, the world has changed again and the average user of today is even further isolated from the internals of a system. And what was a good idea in 1997 may not longer be relevant now.