Humans all need to feel certain about at least some things. We also need to have basic mental and emotional coherence. FIML practice gives partners a very reliable level of mutual certainty and coherence.
Since FIML practice is a process–something you do with your partner–partners will be able to check and recheck their mutual understandings as often as they like. The interpersonal certitude and coherence that result from this process is amazing. It is amazing in and of itself but also because having reliable interpersonal coherence with your partner will have a deep influence on you. It will affect how you understand yourself and how you feel about yourself. It will also affect how you understand and feel about your place in the world, your place in society.
People who do not practice FIML ordinarily get certitude and coherence from outside of themselves–from TV, movies, newspapers, schools, churches, clubs, and so on. The external semiotics of cultures and subcultures created by other people give most individuals the certitude and coherence they need for psychological well-being. Insofar as external semiotics are not sufficient for the individual (and they rarely can do it all), most people fill in whatever is missing with personal interpretations. In many other posts, we have discussed how these personal interpretations are usually based on mistaken impressions. They are usually neurotic, or constitute the kleshas (wrong views, toxic fixations, mistaken interpretations, etc.) described in Buddhist literature.
FIML practice allows partners to correct their neuroses by disconfirming them with their partners. If you disconfirm a neurosis, you effectively confirm that it is/was not true and can therefore be discarded.
FIML practice also helps partners free themselves from the need to find certitude and coherence outside of themselves. As you become more secure in your communication with your partner, both of you will begin to notice that you are becoming less dependent on external semiotics.
FIML emphasizes the certitude and coherence of truth between two caring people above certitude and coherence based on conformance to social norms. FIML helps partners co-form their own subculture rather than conform to a culture created by someone else.