Some of these strategies come from the world of internet chat rooms. There's an honest or even vulnerable quality to this parenthetical expression because you're letting the other person in on something that otherwise could be kept hidden. Of course, people have much more conscious control over these parenthetical actions than they do over body language in the in-person world. Sometimes it's an intentional effort to convey some subtle mood or state of mind. Voice accentuation can be accomplished using caps, asterisks, underlining, and other keyboard characters.
Exclamation points add to the effect.
Accentuating a single word in a sentence sometimes can drastically alter the meaning and impact of the sentence. Rather than highlighting voice, those last two examples above illustrate an action accentuation. On the other hand, I may be wrong. Usually they mimic a pause in one's speaking. It mimics in-person speech patterns.