You are confusing content for expression/listening. It’s like confusing a computer for electricity. That or you are merely misusing the word purpose.

If you want to be better at interpersonal communication you need to be excellent at listening. Empaths find this comes naturally but people with learning disabilities and/or autism will find this impossibly challenging while perhaps convincing themselves of self-mastery.

Listening and expression come in three dimensions: verbal (the words), vocal (use of voice), and physical (the rest of the body). You need to pay attention to all three. Dishonesty is when those three are in conflict. Most people are not good at this and have no idea just how bad they are at it.

Interpersonal communication consists, by definition, of at least two parties. Do you never find it difficult to understand, to be persuaded, to accept feedback, to take on a proposal, to be encouraged, or motivated by someone? If you're only ever the transmitter of those things then I don't think you're actually communicating; you're just orating.

Not to mention that "being friendly", "satisfying interest", and other reasons exist to communicate.

Not everything is about your agenda. Maybe internalising that is hardest?

(A bit pompous but you take my point, right?)

Difficult to grok? We need other people to regulate our own limbic system. Try to think about that one.

I'd say the most difficult are convincing someone of something (persuasion) and giving feedback.

Persuasion is maybe the most difficult outcome to achieve. Giving feedback is the most emotionally challenging and nuanced for both parties. And of course giving feedback is often attempting persuasion too.

Occasionally, just listening and not trying to inject your own intents is hardest.

When someone wants something but chooses not to say it directly. Feedback often fails because someone doesn't want to acknowledge their flaws.