[rbanffy]

PC-DOS inherited it from CP/M 80. Windows was built at first on top of it and then replaced it in stages. I have no idea why would this still be present in the current codebase. It would probably be easier just to fix the filesystem.

[plorkyeran]

There's countless batch files that rely on NUL (including with extensions, usually because some other batch file is appending the extension). Most of the other ones could probably be safely removed, but it would still run contrary to Microsoft's policy of not breaking backwards compatibility unless absolutely necessary.