I like to RP/create characters/plots, and live-RP keeps me focused (as opposed to forum RP which drags on forever and I get distracted). That's.. about it.
Edit: to note you specified Pern. Do you mean PernM* games, or? If it's Pern.. well, I'm selective in the genres I like. It's either this, a couple other small fandoms, or historical RPs. None of them are really all that large. I can find Pern RP easier, IMO.