When people write the sounds they would be making with their mouth.

Starting a sentence with "errr", "uhm", "uhh" on social media or while texting is stupid. I understand that some people may choose to write "hmm" instead of an actual word or sentence depending on the context, like instead of "interesting" or "i understand" but come on.

Writing is not the same as having a face to face conversation, you don't need to write down dumb sounds to make your point. You can't think about what to say for a long time or edit your sentence when you're actually talking to someone, so making filler or mood/thought indicator sounds actually make sense. While you're writing you have time to think and the possibility to make edits, you can form sentences with actual words.