We need Functional Interpersonal Meta Linguistics (FIML) because human communication is fraught with error. Communication errors are unavoidable. They happen very often.
At the most basic level, errors occur because the speaker misspoke or because the listener misheard. Basic errors may involve words, expressions, gestures, or tone of voice.
More complex errors may involve misunderstandings concerning context, intention, or interpretation.
In normal communication, most people do not correct these sorts of errors. Instead, they react to them not as if they were errors but rather the real intention of the speaker. Sometimes, of course, people do attempt to understand and correct communication errors, but most do not do this often enough or with sufficient appreciation for the potential significance of even small errors. Even very basic communication errors can lead people into a mutual spiral of deep misunderstanding.
Functional Interpersonal Meta Linguistics (FIML) is a practice designed to help couples or close friends quickly understand and deal with the errors that inevitably occur in their communications with each other.