It is for fixed length records, which is why it also zeroes the remaining space.

Yes. strncpy was intended for copying file names into a buffer that was only zero terminated when the name was shorter than the maximum length of a file name in Unix (14 bytes. See https://stackoverflow.com/a/1454071, https://devblogs.microsoft.com/oldnewthing/20050107-00/?p=36...)

You can also use it to overwrite part of an existing string, but I think that’s a side effect of the above.

Yes it was. On early Unix systems, each entry in a directory was bascially:

    struct dir
    {
      char name[14];
      int  inode;
    };

Adding a NUL byte might waste a full byte that could otherwise be used---remember, back when C was first developed, 10M disks were large and very expensive.

strncpy was never intended to be a safer strcpy. It was created for a very specialized use case in the Unix kernel--copying a string-ish identifier to a fixed-size char field that only uses NUL termination if the identifier is shorter than the field size. Because of how the C language and the Unix kernel coevolved, it became part of the standard C library by default. I've seen it used for it's original semantics in only a handful of places, but in general it's almost always misused.

To be clear, strncpy does not guarantee NUL termination. It takes a C string as the source argument, but it doesn't write out a C string; it writes out a very esoteric data structure that is unfortunately easily confused with a C string.

By contrast, strlcpy was intended to be a safer string copy routine: https://www.usenix.org/legacy/event/usenix99/full_papers/mil... In particular, it was designed to be what people seem think strncpy is. Its return value semantics are controversial, though mostly only among the glibc crowd as every other Unix libc, including musl and Solaris, now provide it. But the semantics were designed based on experience in fixing old C code, and observations about how developers tend to write C code, not based on prescriptive theories about how people should manipulate C strings in C code.