They are a attempt to replicate, in written form, the rhythm of conversation that is typically encountered when two or more people speak in person, face to face

They are a attempt to replicate, in written form, the rhythm of conversation that is typically encountered when two or more people speak in person, face to face. The "Ummmm" we see in the present example, expresses doubt in what was just said (written, really), and foreshadows that the statement to follow (from speaker #2) may be disagreeable to the other person (speaker #1).' Similarly, it is not uncommon to find the word "Hmmm" as a preface to some statement. This expresses a notion of "I will need to think about this." But in some contexts, "Hmmm" can convey skepticism. In real-life conversation, people frequently exploit this ambiguity, as a clever way to signal that they are skeptical but will not state so out loud.