Although punctually incorrect in the literary world (technically speaking), it's a commonly used and accepted practice with, specifically, playwrights and screenwriters for decades, if not for centuries.
It denotes an emotion and/or emphasis because theatrical productions are not meant to be 'read' per se, they're meant to be an act (in person to person contact).
The internet and its personal interactions (forums, texts, etc) has adopted this practice for the same reason that playwrights and screenwriters have... person to person contact and the need for emotive enunciation when communicating with others.
Internet forums are not writing Pulitzer Prize winning novels, they're interracting with other people.
But you would know this if you didn't get all your 'learnin' from the US public education system.
