I want some defined way to tell the compiler that I am intentionally ignoring the result.

I encounter this when trying to do best-effort logging in a failure path. I call some function to log and error and maybe it fails. If it does, what, exactly, am I going to do about it? Log harder?

Sometimes. For example, you might be setting a non-crucial option on a socket, and if it fails you don't even care to log the fact (maybe the logging would be too expensive), so you just ignore the return value of whatever library is wrapping setsockopt.