One place it's used is to avoid central "registration" for multiple "things". For example, consider a program with many fairly separate "subcommands". Using this mechanism allows adding a new subcommand simply by linking in a new source file (without changing a shared source file to list the new command).

I like to use this technique for CLI command and help tables. It eliminates having a single giant table in a file, instead you have a C macro which makes the system aware of your command right next to your command definition.

   int my_command(..) { ... }
   COMMAND(my_command, "command_name", "example command")
   HELP("use --foo argument for this...")
   HELP("use --bar argument for that...")

Also used this all the time back when I programmed assembly language.

It's definitely a problem that this is not standardized. For example, TI has their own C compiler for their ARM CPUs, not everybody uses gcc..

This is used for constructor & destructor functions in C code. The function pointers get put in an array in a certain section which is collated then the startup and exit code calls them. Ordering is not guaranteed.