PvP characters are only limited to PvP areas. however, you can create customized, maxed out PvP characters instantly, depending on what you have unlocked. you would need to spend quite a bit of effort to max out an RP character.
some people dont wanna do the missions etc 4 times over, and just want to pvp with different classes. so they have one rp char to get the items and skills unlocked/play the story, then have the other 3 for when they want to smash people with a sword, or a spell