because courier is a font that "translates" to all computers. if you try to go with a more modern looking font, you run the risk of the text being broken on someone else's screen.

i personally do it because sometimes [text] in the subject line doesn't cut it. sometimes we chat in the subject line and the [text] gets left behind, sometimes we go a few days between tagging so it's easy to forget, and sometimes people just don't pay the fuck attention. before i started using a different font, i'd had plenty of people mistake text posts for audio posts.