I'm not sure if it actually worked that well in DOS, but in Digital systems, which inspired the DOS shell, it was useful for default file name parts. Using ana

I'm not sure if it actually worked that well in DOS, but in Digital systems, which inspired the DOS shell, it was useful for default file name parts. Using analogy to Unix, the current working directory is useful as a default - to avoid having to retype it. Same with the extension, although with a different semantics.