There’s never really a reason to completely swap mains in Final Fantasy XIV unless you really, really want to have another character. You’re never going to be locked into your job selection or your nation selection; there’s always a chance to go back. On the other hand, it’s very possible to get kind of stuck in games like World of Warcraft. You can change your primary spec, sure, but you can never make your Rogue into a tank, and your Death Knight will never be a healer.
Sometimes it’s just a function of finding yourself playing a game and realizing that your Guild Wars 2 playtime is vastly enhanced when you’re playing a Revenant instead of a Guardian. Or maybe you’ve been playing a Jedi Sentinel forever in Star Wars: The Old Republic, but you’d really rather be a DPS Guardian. Same role, but you can’t remake the choice. And sometimes it’s just a personal choice, like swapping your EVE Online main; sure, you could just train more, but maybe you want a change. So when was the last time you completely changed your main in an MMO? Was it an expansion change, a patch change, or just a change to keep things fresh?