When explaining inodes, directories and hard links, students regularly ask "Why would you (ever) use hard links ?".
I usually reply with three obvious reasons:
1. To never have a file twice on your system (and thus save disk space).
(they need to know that deleting files with hard links doesn't add free space)
2. For compatibility with legacy applications that use old file locations (without the need to rewrite those applications).
3. Because Linux programs know how they are called, mke2fs, mkfs.ext2 and mkfs.ext3 can be the same file, but with distinct functionality.
Sometimes these arguments fail to convince students of the usefulness of hard links. What would you say ?