Set User ID only has a meaning on a file, and it means that if a user runs a program that has SUID set, it will run it as the owner of the program and not as user itself.
That makes SUID efficient for utilities like passwd that allows user to write passwords, but in order to write passwords the user need access to the /etc/shadow file, which is allowed only for user root.
SUID is typically not something that users should apply themselves on files, because it is very dangerous.