C enum tags: When/Why to Use

Question

I'm trying to understand the full syntax of a C enum. For decades, I've successfully used simple enums like the below:

enum {
  IDLE,
  BUSY,
  FAILED
};

But I see the formal definition of a C enum also allows a tag and instance name like the below:

enum STATE {
  IDLE,
  BUSY,
  FAILED
} appstate;

When and why would I ever use "STATE" or "appstate"? For the life of me I can't think of a reason.

Questions 8414188 and 7386805 beat around the bush on this issue but were not enlightening.

UPDATE 5/4/19
I offer some clarifications to my question in response to answers per below:
1. I do use prefixes, e.g. STATE_IDLE, but theses were trimmed in my bare-bones examples above. 2. I have examined several code examples using enums. These confirmed my understanding of the enhanced syntax but did nothing to explain why.
3. I routinely place my anonymous enum in a project-wide .h file, and this seems to work just fine. I still can't figure out why I would want to waste keystrokes to use the politically-correct, enhanced syntax. Is it just for (allegedly) improved readability? Or am I missing something?

Answer 1

You would like to code (and that is actually very common, e.g. enum audit_state in the Linux kernel's kernel/audit.h)

enum STATE {
  IDLE,
  BUSY,
  FAILED
};

^{Such an enum definition is likely to go (by convention) into some public header file. You might adopt the convention to share a common prefix for the various enum values, so write STA_IDLE or STA_BUSY instead of IDLE and BUSY; sometimes I take the convention of suffixing the enum tag, here STATE, with _en so I would rather code enum STATE_en { STA_IDLE, STA_BUSY, ... };}

if you want to use (notably for readability purposes) enum STATE as the type of e.g. some variable or formal:

enum STATE global_state; // a global variable
void print_state(enum STATE); // a formal

or in a typedef:

typedef enum STATE stateT;

etc....

Your appstate is just a variable of type enum STATE (like my global_state is).

I do recommend studying the source code of some existing free software in C (it does not has to be as complex as the Linux kernel; you can find many examples on github).

Don't forget to enable all warnings when compiling (e.g. gcc -Wall -Wextra -g with GCC...), because the compiler might warn you (with explicit enums) in many useful cases (e.g. a switch on that enum missing some case....).

BTW, C programming practically wants a lot of conventions and you might explicit them (e.g. in some coding style guide).